Email Greylisting: An Applicability Statement for SMTP
RFC 6647

Internet Engineering Task Force (IETF)                      M. Kucherawy
Request for Comments: 6647                                     Cloudmark
Category: Standards Track                                     D. Crocker
ISSN: 2070-1721                              Brandenburg InternetWorking
                                                               June 2012

         Email Greylisting: An Applicability Statement for SMTP

Abstract

   This document describes the art of email greylisting, the practice of
   providing temporarily degraded service to unknown email clients as an
   anti-abuse mechanism.

   Greylisting is an established mechanism deemed essential to the
   repertoire of current anti-abuse email filtering systems.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6647.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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Table of Contents

   1. Introduction ....................................................3
      1.1. Background .................................................3
      1.2. Definitions ................................................4
   2. Types of Greylisting ............................................4
      2.1. Connection-Level Greylisting ...............................4
      2.2. SMTP HELO/EHLO Greylisting .................................5
      2.3. SMTP MAIL Greylisting ......................................5
      2.4. SMTP RCPT Greylisting ......................................5
      2.5. SMTP DATA Greylisting ......................................6
      2.6. Additional Heuristics ......................................7
      2.7. Exceptions .................................................7
   3. Benefits and Costs ..............................................8
   4. Unintended Consequences .........................................9
      4.1. Unintended Mail Delivery Failures ..........................9
      4.2. Unintended SMTP Client Failures ...........................10
      4.3. Address Space Saturation ..................................11
   5. Recommendations ................................................12
   6. Measuring Effectiveness ........................................13
   7. IPv6 Applicability .............................................14
   8. Security Considerations ........................................14
      8.1. Trade-Offs ................................................14
      8.2. Database ..................................................14
   9. References .....................................................15
      9.1. Normative References ......................................15
      9.2. Informative References ....................................15
   Appendix A.  Acknowledgments ......................................17

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1.  Introduction

   Preferred techniques for handling email abuse explicitly identify
   good actors and bad actors, giving each significantly different
   service quality.  In some cases, an actor does not have a known
   reputation; this can justify providing degraded service, until there
   is a basis for providing better service.  This latter approach is
   known as "greylisting".  Broadly, the term refers to any degradation
   of service for an unknown or suspect source, over a period of time
   (typically measured in minutes or a small number of hours).  The
   narrow use of the term refers to generation of an SMTP temporary
   failure reply code for traffic from such sources.  There are diverse
   implementations of this basic concept and predictably, therefore,
   some blurred terminology.

   Absent a perfect abuse-detection mechanism that incurs no cost, the
   current requirement is for an array of techniques to be used by each
   filtering system.  They range in cost, effectiveness, and types of
   abuse techniques they target.

   Greylisting happens to be a technique that is cheap and early (in
   terms of its application in the SMTP sequence) and surprisingly
   remains useful.  Some spamware does indeed route around this
   technique, but much does not.

   The firehose of spam over the Internet represents a wide range of
   sophistication.  Greylisting is useful for removing a large amount of
   simplistic-but-significant traffic.

   This memo documents common greylisting techniques and discusses their
   benefits and costs.  It also defines terminology to enable clear
   distinction and discussion of these techniques.

   There is some confusion in the industry that conflates greylisting
   with an SMTP temporary failure for any reason.  The purpose of this
   memo is also to dispel such confusion.

1.1.  Background

   For many years, large amounts of spam have been sent through purpose-
   built software, or "spamware", that supports only a constrained
   version of SMTP.  In particular, such software does not perform
   retransmission attempts after receiving an SMTP temporary failure.
   That is, if the spamware cannot deliver a message, it just goes on to
   the next address in its list since, in spamming, volume counts for
   far more than reliability.  Greylisting exploits this by rejecting
   mail from unfamiliar sources with a "transient (soft) fail" (4xx)
   [SMTP] error code.  Another application of greylisting is to delay

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   mail from newly seen IP addresses on the theory that, if it's a spam
   source, then by the time it retries, it will appear in a list of
   sources to be filtered, and the mail will not be accepted.

   Early references for greylisting descriptions and implementations can
   be found at [SAUCE] and [PUREMAGIC].

1.2.  Definitions

1.2.1.  Keywords

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [KEYWORDS].

1.2.2.  Email Architecture Terminology

   Readers need to be familiar with the material and terminology
   discussed in [MAIL], [EMAIL-ARCH], and [SMTP].

2.  Types of Greylisting

   Greylisting is primarily performed at some phase during an SMTP
   session.  A set of attributes about the client-side SMTP server are
   used for assessing whether to perform greylisting.  At its simplest,
   the attribute is the IP address of the client, and the assessment is
   whether it has previously connected recently.  More elaborate
   attribute combinations and more sophisticated assessments can be
   performed.  The following discussion covers the most common
   combinations and relies on knowledge of [SMTP], its commands, and the
   distinction between envelope and content.

2.1.  Connection-Level Greylisting

   Connection-level greylisting decides whether to accept the TCP
   connection from a "new" [SMTP] client.  At this point in the
   communication between the client and the server, the only information
   known to the receiving server is the incoming IP address.  This, of
   course, is often (but not always) translatable into a host name.

   The typical application of greylisting here is to keep a record of
   SMTP client IP addresses and/or host names (collectively, "sources")
   that have been seen.  Such a database acts as a cache of known
   senders and might or might not expire records after some period.  If
   the source is not in the database, or the record of the source has
   not reached some required minimum age (such as 30 minutes since the
   initial connection attempt), the server does one of the following,
   inviting a later retry:

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   o  returns a 421 SMTP reply and closes the connection, or

   o  returns a different 4yz SMTP reply to all further commands in this
      SMTP session.

   A useful variant of the basic known/unknown policy is to limit
   greylisting to those addresses that are on some list of IP addresses
   known to be affiliated with bad actors.  Whereas the simpler policy
   affects all new connections, including those from good actors, the
   constrained policy applies greylisting actions only to sites that
   already have a negative reputation.

2.2.  SMTP HELO/EHLO Greylisting

   HELO/EHLO greylisting refers to the first command verb in an SMTP
   session.  It includes a single, required parameter that is supposed
   to contain the client's fully qualified host name or its literal IP
   address.

   Greylisting implemented at this phase retains a record of sources
   coupled with HELO/EHLO parameters.  It returns 4yz SMTP replies to
   all commands until the end of the SMTP session if that tuple has not
   previously been recorded or if the record exists but has not reached
   some configured minimum age.

2.3.  SMTP MAIL Greylisting

   MAIL command greylisting refers to the command verb in an SMTP
   session that initiates a new transaction.  It includes at least one
   required parameter that indicates the return email address
   (RFC5321.MailFrom) of the message being relayed from the client to
   the server.

   Greylisting implemented at this phase retains a record of sources
   coupled with return email addresses.  It returns 4yz SMTP replies to
   all commands for the remainder of the SMTP session if that tuple has
   not previously been recorded or if the record exists but has not met
   some configured minimum age.

2.4.  SMTP RCPT Greylisting

   RCPT greylisting refers to the command verb in an SMTP session that
   specifies intended recipients of an email transaction.  It includes
   at least one required parameter that indicates the email address of
   an intended recipient of the message being relayed from the client to
   the server.

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   Greylisting implemented at this phase retains a record of tuples that
   combines the provided recipient address with any combination of the
   following:

   o  the source, as described above;

   o  the return email address; and

   o  the other recipient addresses of the message (if any).

   If the selected tuple is not found in the database, or if the record
   is present but has not reached some configured minimum age, the
   greylisting Mail Transfer Agent (MTA) [EMAIL-ARCH] returns 4yz SMTP
   replies to all commands for the remainder of the SMTP session.

   Note that often a match on a tuple involving the first valid RCPT is
   sufficient to identify a retry correctly, and further checks can be
   omitted.

2.5.  SMTP DATA Greylisting

   DATA greylisting refers to the command verb in an SMTP session that
   transmits the actual message content, as opposed to its envelope
   details.

   This type of greylisting can be performed at two places in the SMTP
   sequence:

   1.  on receipt of the DATA command, because at that point the entire
       envelope has been received (i.e., all MAIL and RCPT commands have
       been issued); or

   2.  on completion of the DATA command, i.e., after the "." that
       terminates transmission of the message body, since at that point
       a digest or other analysis of the message could be performed.

   Some implementations do filtering here because there are clients that
   don't bother checking SMTP reply codes to commands other than DATA.
   Hence, it can be useful to add greylisting capability at that point
   in an SMTP session.

   Numerous greylisting policies are possible at this point.  All of
   them retain a record of tuples that combine the various parts of the
   SMTP transaction in some combination, including:

   o  the source, as described above;

   o  the return email address;

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   o  the recipients of the message, as a set or individually;

   o  identifiers in the message header, such as the contents of the
      RFC5322.From or RFC5322.To fields;

   o  other prominent parts of the content, such as the RFC5322.Subject
      field;

   o  a digest of some or all of the message content, as a test for
      uniqueness; and

   o  analysis of arbitrary portions of the message body.

   (The last four items in the list above are only possible at the end
   of DATA, not on receipt of the DATA command.)

   If the selected tuple is not found in the database, or if the record
   exists but has not reached some configured minimum age, the
   greylisting MTA returns 4yz SMTP replies to all commands for the
   remainder of the SMTP session.

2.6.  Additional Heuristics

   Since greylisting seeks to target spam senders, it follows that being
   able to identify spamware within the SMTP context beyond the simple
   notion of "not seen before" would be desirable.  A more targeted
   approach might also include in its selection heuristics such as the
   following:

   o  If a DNS blacklist [DNSBL] lists an IP address but the implementer
      wishes to be cautious with mitigation actions rather than blocking
      traffic from the IP address outright, then subject it to
      greylisting.

   o  If the value found in a PTR record follows common naming patterns
      for dynamic IP addresses, then subject it to greylisting.

2.7.  Exceptions

   Most greylisting systems provide for an exception mechanism, allowing
   one to specify IP addresses, IP address Classless Inter-Domain
   Routing (CIDR) [CIDR] blocks, host names, or domain names that are
   exempt from greylisting checks and thus whose SMTP client sessions
   are not subject to such interference.

   Likely candidates to be excepted from greylisting include those known
   not to retry according to a pattern that will be observed as
   legitimate and those that send so rarely that they will age out of

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   the database.  In both cases, the excepted source is known not to be
   an abusive one by the site implementing greylisting.  Otherwise,
   typical non-abusive senders will enter the exception list on the
   first proper retry and remain there permanently.

   One could also use a [DNSBL] that lists known good hosts as a
   greylisting exception set.

3.  Benefits and Costs

   The most obvious benefit with any of the above techniques is that
   spamware generally does not retry and is therefore less likely to
   succeed, absent a record of a previous delivery attempts.

   The most obvious detriment to implementing greylisting is the
   imposition of delay on legitimate mail.  Some popular MTAs do not
   retry failed delivery attempts for an hour or more, which can cause
   expensive delays when delivery of mail is time critical.  Worse, some
   legitimate MTAs do not retry at all.  (Note, however, that non-
   retrying clients are not fully SMTP-capable, per Section 2.1 of
   [SMTP].  A client does not know, nor is it entitled to know, the
   reason for the temporary failure status code being returned;
   greylisting could be in effect, or it could be caused by a local
   resource issue at the server.  A client therefore needs to be
   equipped to retry in order to be considered fully capable.)

   The counterargument to this "false positive" problem is that email
   has always been a "best-effort" mechanism; thus, this cost is
   ultimately low in comparison to the cost of dealing with high volumes
   of unwanted mail.  Still, the actual effect of such delays can be
   significant, such as altering the tone or flow of a multi-participant
   discussion to a mailing list.

   When the clients are subjected to any kind of reconfiguration,
   especially network renumbering, the cache of information stored about
   SMTP client history does not benefit legitimate clients that are
   already listed for acceptance.  To the greylisting implementation,
   such clients are once again unknown, and they will once again be
   subjected to the delay.

   Another obvious cost is for the required database.  It has to be
   large enough to keep the necessary history and fast enough to avoid
   excessive inefficiencies in the server's operations.  The primary
   consideration is the maximum age of records in the database.  If
   records age out too soon, then hosts that do retry per [SMTP] will be
   periodically subjected to greylisting even though they are well-

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   behaved; if records age out after too long a period, then eventually
   spamware that launches a new campaign will not be identified as
   "unknown" in this manner and will not be required to retry.

   Presuming that known friendly senders will be manually configured as
   exceptions to the greylisting check, a steady state will eventually
   be reached wherein the only mail that is delayed is mail from an IP
   address that has never sent mail before.  Experience suggests that
   the vast majority of mail comes from places on a developed exception
   list, so after a training period, only a small proportion of mail is
   actually affected.  The training period could be replaced by
   processing a history of email traffic and adding the IP addresses
   from which most traffic arrives to the exception list.

   Applying greylisting based on actual message content (i.e., post-
   DATA) is substantially more expensive than any of the other
   alternatives both in terms of the resources required to accept and
   temporarily store a complete message body (which can be quite
   substantial) and any processing that is done on that content.  As a
   consequence, such methods incur more cost during the session and thus
   are not typical practice.

4.  Unintended Consequences

4.1.  Unintended Mail Delivery Failures

   There are a few failure modes of greylisting that are worth
   considering.  For example, consider an email message intended for
   user@example.com.  The example.com domain is served by two receiving
   mail servers, one called mail1.example.com and one called
   mail2.example.com.  On the first delivery attempt, mail1.example.com
   greylists the client, and thus the client places the message in its
   outgoing queue for later retry.  Later, when a retry is attempted,
   mail2.example.com is selected for the delivery, either because
   mail1.example.com is unavailable or because a round-robin [DNS]
   evaluation produces that result.  However, the two example.com hosts
   do not share greylisting databases, so the second host again denies
   the attempt.  Thus, although example.com has sought to improve its
   email throughput by having two servers, it has, in fact, amplified
   the problem of legitimate mail delay introduced by greylisting.

   Similarly, consider a site with multiple outbound MTAs that share a
   common queue.  On a first outbound delivery attempt to example.com,
   the attempt is greylisted.  On a later retry, a different outbound
   MTA is selected, which means example.com sees a different source, and
   once again greylisting occurs on the same message.  The same effect
   can result from the use of [DHCP], where the IP address of an
   outbound MTA changes between attempts.

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   For systems that do DATA-level greylisting, if any part of the
   message has changed since the first attempt, the tuple constructed
   might be different than the one for the first attempt, and the
   delivery is again greylisted.  Some MTAs do reformulate portions of
   the message at submission time, and this can produce visible
   differences for each attempt.

   A host that sends mail to a particular destination infrequently might
   not remain "known" in the receiving server's database and will
   therefore be greylisted for a high percentage of mail despite
   possibly being a legitimate sender.

   All of these and other similar cases can cause greylisting to be
   applied improperly to legitimate MTAs multiple times, leading to long
   delays in delivery or ultimately the return of the message to its
   sender.  Other side effects include out-of-order delivery of related
   sequenced messages.

   Address translation technologies such as [NAT] cause distinct MTAs to
   appear to come from a common IP address.  This can cause greylisting
   to be applied only to the first connection attempt from the shared IP
   address, meaning future MTAs connecting for the first time will be
   exempted from the protection greylisting provides.

4.2.  Unintended SMTP Client Failures

   Atypical SMTP client behaviors also need to be considered when
   deploying greylisting.

   Some clients do not retry messages for very long periods.  Popular
   open source MTAs implement increasing backoff times when messages
   receive temporary failure messages and/or degrade queue priority for
   very large messages.  This means greylisting introduces even more
   delay for MTAs implementing such schemes, and the delay can become
   large enough to become a nuisance to users.

   Some clients do not retry messages at all, in violation of [SMTP].
   This means greylisting will cause outright delivery failure right
   away for sources, envelopes, or messages that it has not seen before,
   regardless of the client attempting the delivery, essentially
   treating legitimate mail and spam the same.

   If a greylisting scheme requires a database record to have reached a
   certain age rather than merely testing for the presence of the record
   in the database, and the client has a retry schedule that is too
   aggressive, the client could be subjected to rate limiting by the MTA
   independent of the restrictions imposed by greylisting.

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   Some SMTP implementations make the error of treating all error codes
   as fatal, contrary to [SMTP]; that is, a 4yz response is treated as
   if it were a 5yz response, and the message is returned to the sender
   as undeliverable.  This can result in such things as inadvertent
   removal from mailing lists in response to the perceived rejections.

   Some clients encode message-specific details in the address parameter
   to the [SMTP] MAIL command.  If doing so causes the parameter to
   change between retry attempts, a greylisting implementation could see
   it as a new delivery rather than a retry and disallow the delivery.
   In such cases, the mail will never be delivered and will be returned
   to the sender after the retry timeout expires.

   A client subjected to greylisting might move to the next host found
   in the ordered [DNS] MX record set for the destination domain and re-
   attempt delivery.  This has several considerations of its own:

   o  Traffic to those alternate servers increases merely as a result of
      greylisting.

   o  Alternate (MX) servers SHOULD share the same greylisting database.
      When they do not -- as is often true when the servers occupy
      different Administrative Management Domains (ADMDs) -- SMTP
      clients can see variable treatment if they try to send to
      different MX hosts.

   o  When alternate MX servers relay mail back to the "primary" MX
      server, the latter SHOULD be configured to permit the other
      servers to relay mail without being subjected to greylisting.

   There are some applications that connect to an SMTP server and
   simulate a transaction up to the point of sending the RCPT command in
   an attempt to confirm that an address is valid.  Some of these are
   legitimate applications (e.g., mailing list servers), and others are
   automated programs that attempt to ascertain valid addresses to which
   to send spam (a "directory harvesting" attack).  Greylisting can
   interfere with both instances, with harmful effects on the former.

4.3.  Address Space Saturation

   Greylisting is obviously not a foolproof solution to avoiding abusive
   traffic.  Bad actors that send mail with just enough frequency to
   avoid having their records expire will never be caught by this
   mechanism after the first instance.

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   Where this is a concern, combining greylisting with some form of
   reputation service that estimates the likely behavior for IP
   addresses that are not intercepted by the greylisting function would
   be a good choice.

5.  Recommendations

   The following practices are RECOMMENDED based on collected
   experience:

   1.  Implement greylisting based on a tuple consisting of (IP address,
       RFC5321.MailFrom, and the first RFC5321.RcptTo).  It is
       sufficient to use only the first RFC5321.RcptTo as legitimate
       MTAs appear not to reorder recipients between retries.  Including
       RFC5321.MailFrom improves accuracy where the IP address is being
       matched in clusters (e.g., CIDR blocks) rather than precisely
       (see below).  After a successful retry, allow all further [SMTP]
       traffic from the IP address in that tuple regardless of envelope
       information.

   2.  Include a configurable range of time within which a retry from a
       greylisted host is considered and outside of which it is
       otherwise ignored.  The range needs to cover typical retry times
       of common MTA configurations, thus anticipating that a fully
       capable MTA will retry sometime after the beginning of the range
       and before the end of it.  The default range SHOULD be from one
       minute to 24 hours.  Retries within the range are permitted and
       satisfy the greylisting test, and the client is thus no longer
       likely to be a sender of spam.  Retries after the end of the
       range SHOULD be considered to be a new message for the purposes
       of greylisting evaluation (i.e., reset the "first seen" timestamp
       for that IP address).  Some sites use a higher time value for the
       low end of the time range to match common legitimate MTA retry
       timeouts, but additional benefit from doing so appears unlikely.

   3.  Include a timeout for database entries, after which records for
       IP addresses that have generated no recent traffic are deleted.
       This step is intended to re-enable greylisting for an IP address
       in the event that it has changed "owners" and will subject the
       client to another round of greylisting.  The default SHOULD be at
       least one week.

   4.  For an Administrative Management Domain (ADMD), all inbound
       border MTAs listed in the [DNS] SHOULD share a common greylisting
       database and common greylisting policies.  This handles sequences
       in which a client's retry goes to a different server after the
       first 4yz reply, and it lets all servers share the list of hosts
       that did retry successfully.

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   5.  To accommodate those senders that have clusters of outgoing mail
       servers, greylisting servers MAY track CIDR blocks of a size of
       its own choosing, such as /24, rather than the full IPv4 address.
       (Note, however, that this heuristic will not work for clusters
       having machines on different networks.)  A similar grouping
       capability MAY be established based on the domain name of the
       mail server if one can be determined.

   6.  Include a manual override capability for adding specific IP
       addresses or network blocks that always bypass checks.  There are
       legitimate senders that simply don't respond well to greylisting
       for a variety of reasons, most of which do not conflict with
       [SMTP].  There are also some highly visible online entities such
       as email service providers that will be certain to retry; thus,
       those that are known SHOULD be allowed to bypass the filter.

   7.  Greylisting SHOULD NOT be applied by an ADMD's submission service
       (see [SUBMISSION]) for authenticated client hosts.  It also
       SHOULD not be applied against any authenticated ADMD session.
       Authentication can include whatever mechanisms are deemed
       appropriate for the ADMD, such as known internal IP addresses,
       protocol-level client authentication, or the like.

   There is no specific recommendation as to the specific choice of 4yz
   code to be returned as a result of a greylisting delay.  Per [SMTP],
   however, the only two reasonable choices are 421 if the
   implementation wishes to terminate the connection immediately and 450
   otherwise.  It is possible that some clients treat different 4yz
   codes differently, but no data is available on whether using 421
   versus some other 4yz code is particularly advantageous.

   There is also no specific recommendation as to the choice of text to
   include in the SMTP reply, if any.  Some implementers argue that
   indicating that greylisting is in effect can give spamware a hint as
   to when to try again for successful delivery, while others suspect
   that it won't matter to spamware and thus the more likely audience is
   legitimate senders seeking to understand why their mail is being
   delayed.

6.  Measuring Effectiveness

   A few techniques are common when measuring the effectiveness of
   greylisting in a particular installation:

   o  Arrange to log the spam versus legitimate determinations of
      messages and what the greylisting decision would have been if
      enabled; then determine whether there is a correlation (and, of
      course, whether too much legitimate email would also be affected).

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   o  Continuing from the previous point, query the set of IP addresses
      subjected to greylisting in any popular [DNSBL] to see if there is
      a strong correlation.

7.  IPv6 Applicability

   The descriptions and recommendations presented in this memo are based
   on many years of experience with greylisting in the IPv4 Internet
   environment, so they clearly pertain to IPv4 deployments only.

   The greater size of an IPv6 address seems likely to permit
   differences in behaviors by bad actors, and this could well mean
   needing to alter the details for applying greylisting; it might even
   negate any benefits in using greylisting at all.  At a minimum, it is
   likely to call for different specific choices for any greylisting
   algorithm variables.

   In addition, an obvious consideration is that the size of the
   database required to store records of all of the IP addresses seen
   will likely be substantially larger in the IPv6 environment.

8.  Security Considerations

   This section discusses potential security issues related to
   greylisting.

8.1.  Trade-Offs

   The discussion above highlights the fact that, although greylisting
   provides some obvious and valuable defenses, it can introduce
   unintentional and detrimental consequences for delivery of legitimate
   mail.  Where timely delivery of email is essential, especially for
   financial, transactional, or security-related applications, the
   possible consequences of such systems need to be carefully
   considered.

   Specific sources can be exempted from greylisting, but, of course,
   that means they have elevated privilege in terms of access to the
   mailboxes on the greylisting system, and malefactors can seek to
   exploit this.

8.2.  Database

   The database that has to be maintained as part of any greylisting
   system will grow as the diversity of its SMTP clients' hosts grows
   and, of course, is larger in general depending on the nature of the
   tuple stored about each delivery attempt.  Even with a record aging
   policy in place, such a database could grow large enough to interfere

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   with the system hosting it, or at least to a point at which
   greylisting service is degraded.  Moreover, an attacker knowing which
   greylisting scheme is in use could rotate parameters of SMTP clients
   under its control, in an attempt to inflate the database to the point
   of denial-of-service.

   Implementers could consider configuring an appropriate failure policy
   so that something locally acceptable happens when the database is
   attacked or otherwise unavailable.

   In practice, this has not appeared as a serious concern, because any
   reasonable aging policy successfully moderates database growth.  It
   is nevertheless identified here as a consideration as there may be
   implementations in some environments where this is indeed an issue.

9.  References

9.1.  Normative References

   [EMAIL-ARCH]  Crocker, D., "Internet Mail Architecture", RFC 5598,
                 July 2009.

   [KEYWORDS]    Bradner, S., "Key words for use in RFCs to Indicate
                 Requirement Levels", BCP 14, RFC 2119, March 1997.

   [SMTP]        Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
                 October 2008.

   [SUBMISSION]  Gellens, R. and J. Klensin, "Message Submission for
                 Mail", STD 72, RFC 6409, November 2011.

9.2.  Informative References

   [CIDR]        Fuller, V. and T. Li, "Classless Inter-domain Routing
                 (CIDR): The Internet Address Assignment and Aggregation
                 Plan", BCP 122, RFC 4632, August 2006.

   [DHCP]        Droms, R., "Dynamic Host Configuration Protocol",
                 RFC 2131, March 1997.

   [DNS]         Mockapetris, P., "Domain names - implementation and
                 specification", STD 13, RFC 1035, November 1987.

   [DNSBL]       Levine, J., "DNS Blacklists and Whitelists", RFC 5782,
                 February 2010.

   [MAIL]        Resnick, P., Ed., "Internet Message Format", RFC 5322,
                 October 2008.

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   [NAT]         Srisuresh, P. and K. Egevang, "Traditional IP Network
                 Address Translator (Traditional NAT)", RFC 3022,
                 January 2001.

   [PUREMAGIC]   Harris, E., "The Next Step in the Spam Control War:
                 Greylisting", August 2003,
                 <http://projects.puremagic.com/greylisting/
                 whitepaper.html>.

   [SAUCE]       Jackson, I., "GNU SAUCE", 2001,
                 <http://www.gnu.org/software/sauce>.

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Appendix A.  Acknowledgments

   The authors wish to acknowledge Mike Adkins, Steve Atkins, Mihai
   Costea, Derek Diget, Peter J. Holzer, John Levine, Chris Lewis, Jose-
   Marcio Martins da Cruz, John Klensin, S. Moonesamy, Suresh
   Ramasubramanian, Mark Risher, Jordan Rosenwald, Gregory Shapiro, Joe
   Sniderman, Roland Turner, and Michael Wise for their contributions to
   this memo.  The various participants of the MAAWG Open Sessions about
   greylisting were also valued contributors.

Authors' Addresses

   Murray S. Kucherawy
   Cloudmark
   128 King St., 2nd Floor
   San Francisco, CA  94107
   US

   Phone: +1 415 946 3800
   EMail: superuser@gmail.com

   Dave Crocker
   Brandenburg InternetWorking
   675 Spruce Dr.
   Sunnyvale, CA  94086
   USA

   Phone: +1.408.246.8253
   EMail: dcrocker@bbiw.net
   URI:   http://bbiw.net

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