INTERNET-DRAFT Viruthagiri Thirumavalavan
Intended Status: Proposed Standard Dombox, Inc.
Expires: May 26, 2020 Nov 26, 2019
Email Address Length
draft-viruthagiri-email-address-length-00
Abstract
This memo formally defines the overall email address maximum length
to 254 characters (octets);
Relaxes local-part limitation set by RFC 821/2821/5321;
Obsoletes domain-part limitation set by RFC 821/2821/5321; AND
Uses the mechanism described in RFC 1869 to define an extension with
keyword "EAML" to the SMTP service whereby an SMTP server can declare
that it is capable of handling longer email addresses.
Status of this Memo
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Copyright and License Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2 Background . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Maximum Length . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Minimum Length . . . . . . . . . . . . . . . . . . . . . . . . 5
5. Obsoleting domain-part limitation . . . . . . . . . . . . . . . 6
6. Relaxing local-part limitation . . . . . . . . . . . . . . . . 7
6.1. Variable Envelope Return Path (VERP) . . . . . . . . . . . 7
6.2. Sender Rewriting Scheme (SRS) . . . . . . . . . . . . . . . 9
6.3. Receiving Servers . . . . . . . . . . . . . . . . . . . . . 10
6.4. Clients . . . . . . . . . . . . . . . . . . . . . . . . . . 11
6.5. Form Inputs . . . . . . . . . . . . . . . . . . . . . . . . 12
7. Framework for the Email Address Maximum Length Extension . . . 12
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 14
9. Security Considerations . . . . . . . . . . . . . . . . . . . . 14
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
10.1. Normative References . . . . . . . . . . . . . . . . . . . 14
10.2. Informative References . . . . . . . . . . . . . . . . . . 15
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15
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1. Definitions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC2119].
The term "character" in this document refers to an "octet character".
2 Background
[RFC3696], an informational RFC says:
+-------------------------------------------------------------------+
|In addition to restrictions on syntax, there is a length limit on |
|email addresses. That limit is a maximum of 64 characters (octets) |
|in the "local part" (before the "@") and a maximum of 255 |
|characters (octets) in the domain part (after the "@") for a total |
|length of 320 characters. Systems that handle email should be |
|prepared to process addresses which are that long, even though they|
|are rarely encountered. |
+-------------------------------------------------------------------+
Both [RFC821] and [RFC2821] says:
+-------------------------------------------------------------------+
|The maximum total length of a reverse-path or forward-path is 256 |
|characters (including the punctuation and element separators). |
+-------------------------------------------------------------------+
[RFC5321] updated that part with the following text:
+-------------------------------------------------------------------+
|The maximum total length of a reverse-path or forward-path is 256 |
|octets (including the punctuation and element separators). |
+-------------------------------------------------------------------+
To comply with [RFC5321] / [RFC2821] / [RFC821], John C. Klensin, the
author of RFC3696, filed Errata [1003] with the following text:
+-------------------------------------------------------------------+
|In addition to restrictions on syntax, there is a length limit on |
|email addresses. That limit is a maximum of 64 characters (octets) |
|in the "local part" (before the "@") and a maximum of 255 |
|characters (octets) in the domain part (after the "@") for a total |
|length of 320 characters. However, there is a restriction in |
|RFC 2821 on the length of an address in MAIL and RCPT commands of |
|256 characters. Since addresses that do not fit in those fields are|
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|not normally useful, the upper limit on address lengths should |
|normally be considered to be 256. |
+-------------------------------------------------------------------+
There is one issue however. [RFC5321] / [RFC2821] / [RFC821] places
the 256 character limit on the path. A path is defined as
Path = "<" [ A-d-l ":" ] Mailbox ">"
So the forward-path will contain at least a pair of angle brackets in
addition to the Mailbox. This limits the Mailbox (i.e. the email
address) to 254 characters.
To correct that issue, Dominic Sayers filed Errata [1690] and updated
that part with the following text:
+-------------------------------------------------------------------+
|In addition to restrictions on syntax, there is a length limit on |
|email addresses. That limit is a maximum of 64 characters (octets)|
|in the "local part" (before the "@") and a maximum of 255 |
|characters (octets) in the domain part (after the "@") for a total |
|length of 320 characters. However, there is a restriction in |
|RFC 2821 on the length of an address in MAIL and RCPT commands of |
|254 characters. Since addresses that do not fit in those fields are|
|not normally useful, the upper limit on address lengths should |
|normally be considered to be 254. |
+-------------------------------------------------------------------+
3. Maximum Length
The Maximum Length for an email address is
+-------------------------------------------------------------------+
| |
| 254 Characters |
| |
+-------------------------------------------------------------------+
Refer Section 2 for more info.
This Maximum Length is a "HARD LIMIT" for end user form inputs. e.g.
Websites, Mobile Apps, Gaming Consoles, Smart TVs, Smart Watches,
Printers, DSLR cameras etc.
But this is a "SOFT LIMIT" for Receiving Servers. i.e. A receiving
server MUST support AT LEAST 254 maximum character email address. A
receiving server can extend this limit with the help of EAML SMTP
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extension. EAML SMTP extension is defined in Section 7.
4. Minimum Length
The Minimum Length for an email address is
+-------------------------------------------------------------------+
| |
| 5 Characters |
| |
+-------------------------------------------------------------------+
The "Minimum Length" in this section applicable only to DNS-based
email addresses. i.e. The sender performs a DNS lookup to fetch MX
records before transmitting the mail.
The minimum length in this section applicable only to end user form
inputs.
_INFORMATIONAL_:
The term "email address" is a generic term. It can refer to an
Internet address, Intranet address, X.400 address etc.
Internet Address:
An email address on the Internet would look like a@b.c
e.g. john@example.com [user@domain]
Intranet Address:
An email address on the Intranet may look like a@b
e.g. alice@machine50 [user@host]
X.400 Address:
An X.400 address is technically referred to as an
Originator/Recipient (OR) address. It consists of several
elements, including:
C (Country name)
ADMD (Administration Management Domain, short-form A),
usually a public mail service provider
PRMD (Private Management Domain, short-form P)
O (Organization name)
OU (Organizational Unit Names), OU is equivalent to OU0,
can have OU1, OU2...
G (Given name)
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I (Initials)
S (Surname)
e.g. G=Harald;S=Alvestrand;O=Uninett;PRMD=Uninett;A=;C=no
IP address Literal:
jsmith@[192.168.2.1]
jsmith@[IPv6:2001:db8::1]
5. Obsoleting domain-part limitation
[RFC821] says:
+-------------------------------------------------------------------+
|The maximum total length of a domain name or number is 64 |
|characters. |
+-------------------------------------------------------------------+
[RFC2821] updated that part with the following text:
+-------------------------------------------------------------------+
|The maximum total length of a domain name or number is 255 |
|characters. |
+-------------------------------------------------------------------+
[RFC5321] updated that part again with the following text:
+-------------------------------------------------------------------+
|The maximum total length of a domain name or number is 255 |
|octets. |
+-------------------------------------------------------------------+
Since the maximum total length of an email address is 254 characters,
the domain-part limitation of 255 characters is flawed and redundant.
It's flawed because, any such limitation should be less than overall
email address length.
i.e. domain-part length < overall email address length
It's redundant because, any email address that has 255 domain
characters already an invalid email address due to overall email
address length 254 characters.
So this document obsoletes that limitation.
If overall email address length can be defined as "n", then the
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maximum total length of a domain name or number is "n-2" characters.
We allocated 1 character for the "@" symbol and 1 character for the
minimum possible local-part length.
6. Relaxing local-part limitation
[RFC821] says:
+-------------------------------------------------------------------+
|The maximum total length of a user name is 64 characters. |
+-------------------------------------------------------------------+
[RFC2821] updated that part with the following text:
+-------------------------------------------------------------------+
|The maximum total length of a user name or other local-part is 64 |
|characters. |
+-------------------------------------------------------------------+
[RFC5321] updated that part again with the following text:
+-------------------------------------------------------------------+
|The maximum total length of a user name or other local-part is 64 |
|octets. |
+-------------------------------------------------------------------+
This document relaxes the local-part limitation for the reasons found
in section 6.1 and 6.2
6.1. Variable Envelope Return Path (VERP)
Most bounce messages have historically been designed to be read by
human users, not automatically handled by software. They all convey
the same basic idea (the message from X to Y could not be delivered
because of reason Z) but with so many variations that it would be
nearly impossible to write a program to reliably interpret the
meaning of every bounce message. RFC 1894 (obsoleted by RFC 3464)
defines a standard format to fix this problem, but support for the
standard is far from universal.
[VERP] solves the bounce handling problem.
The hard part of bounce handling is matching up a bounce message with
the undeliverable address that caused the bounce. If the mailing list
software can see that a bounce resulted from an attempt to send a
message to user@example.com then it doesn't need to understand the
rest of the information in the bounce. It can simply count how many
messages were recently sent to user@example.com, and how many bounces
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resulted, and if the proportion of bounced messages is too high, the
address is removed from the list.
While bounce message formats in general vary wildly, there is one
aspect of a bounce message that is highly predictable: the address to
which it will be sent. VERP takes full advantage of this. In a
mailing list that uses VERP, a different MAIL FROM address is used
for each recipient.
The mailing list manager knows that it sent a message from X (MAIL
FROM) to Y (RCPT TO), so if a bounce message is received at address X
(MAIL FROM), it can only be because address Y (RCPT TO) was
undeliverable, because nothing was sent from X (MAIL FROM) to any
other address. Thus the important information has been extracted from
the bounce message, without any need to understand its contents,
which means the person in charge of the list does not need to deal
with it manually.
Typically, an email from Alice to Bob in the above example will have
headers like the following:
+-------------------------------------------------------------------+
|From: alice@example.org |
|To: bob@example.com |
|Return-Path: bounces@example.org |
+-------------------------------------------------------------------+
A very simple VERP address for a mail to bob@example.com will be:
+-------------------------------------------------------------------+
|MAIL FROM:<bounces-bob=example.com@example.org> |
+-------------------------------------------------------------------+
bob is a 3 character local-part, so there won't be any issues.
Let's just assume a user with 62 character local-part subscribe to
the list.
A normal mail would look like this.
+-------------------------------------------------------------------+
|From: alice@mailchimp.com |
|To: this-electronic-mail-address-local-part- |
| contains-62-characters@example.com |
|Return-Path: bounces@example.org |
+-------------------------------------------------------------------+
The [VERP] address would look like:
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+-------------------------------------------------------------------+
|MAIL FROM:<bounces-this-electronic-mail-address-local-part- |
| contains-62-characters=example.com@example.org> |
+-------------------------------------------------------------------+
Above [VERP] address local-part contains 82 characters. If
example.com sticks to the RFC limit 64 characters, then the above
VERP address structure won't work.
[VERP] was introduced by Daniel J. Bernstein in 1997 and today many
software supports VERP.
To name a few,
AmazonSES, CiviCRM, Courier Mail Server, Discourse, exim, ezmlm, GNU
Mailman, Inxmail, Mercury Mail Transport System, mlmmj, Mahara,
Mailchimp, MediaWiki, Moodle, postfix, qmail, Sendmail, STEdb,
StrongMail, Sympa, Thexyz, Zimbra, Target Box, NotifyBC
6.2. Sender Rewriting Scheme (SRS)
Sender Rewriting Scheme ([SRS]) is a scheme for rewriting the
envelope sender address of an email message, in view of remailing it.
In this context, remailing is a kind of email forwarding. SRS was
devised in order to forward email without breaking the Sender Policy
Framework (SPF), back in 2003.
[SRS] is a form of variable envelope return path ([VERP]) inasmuch as
it encodes the original envelope sender in the local part of the
rewritten address. Consider example.com forwarding a message
originally destined to bob@example.com to his new address
<bob@example.net>
+-------------------------------------------------------------------+
|ORIGINAL |
|envelope sender: alice@example.org |
|envelope recipient: bob@example.com |
| |
|REWRITTEN |
|envelope sender: SRS0=HHH=TT=example.org=alice@example.com |
|envelope recipient: bob@example.net |
+-------------------------------------------------------------------+
With respect to VERP, the local part (alice) is moved after her
domain name (example.org), further adding a prefix (SRS0), a hash
(HHH), and a timestamp (TT)
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SRS provides for another prefix, SRS1, to be used for rewriting an
already rewritten address, in a multi-hop scenario. If example.net
has to forward the message in turn, it can spare adding another
timestamp and repeating the original local part (alice). That is,
each new forwarder adds just its own hash (HHH) and the domain name
of the preceding forwarder:
+-------------------------------------------------------------------+
|FURTHER REWRITTEN |
|envelope sender: SRS1=HHH=example.com==HHH=TT=example.org |
| =alice@example.net |
|envelope recipient: bob@further.example |
+-------------------------------------------------------------------+
SRS1 incorporates 2 domains in the local-part. So 64 characters are
not enough.
6.3. Receiving Servers
[RFC821], Section 4.5.3. says:
+-------------------------------------------------------------------+
|There are several objects that have required minimum maximum |
|sizes. That is, every implementation must be able to receive |
|objects of at least these sizes, but must not send objects |
|larger than these sizes. |
+-------------------------------------------------------------------+
+-------------------------------------------------------------------+
| |
| TO THE MAXIMUM EXTENT POSSIBLE, IMPLEMENTATION |
| TECHNIQUES WHICH IMPOSE NO LIMITS ON THE LENGTH |
| OF THESE OBJECTS SHOULD BE USED. |
| |
+-------------------------------------------------------------------+
[RFC2821], Section 4.5.3.1 says:
+-------------------------------------------------------------------+
|There are several objects that have required minimum/maximum sizes.|
|Every implementation MUST be able to receive objects of at least |
|these sizes. Objects larger than these sizes SHOULD be avoided when|
|possible. However, some Internet mail constructs such as encoded |
|X.400 addresses will often require larger objects: clients MAY |
|attempt to transmit these, but MUST be prepared for a server to |
|reject them if they cannot be handled by it. To the maximum extent |
|possible, implementation techniques which impose no limits on the |
|length of these objects should be used. |
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+-------------------------------------------------------------------+
[RFC5321], Section 4.5.3.1 says:
+-------------------------------------------------------------------+
|There are several objects that have required minimum/maximum sizes.|
|Every implementation MUST be able to receive objects of at least |
|these sizes. Objects larger than these sizes SHOULD be avoided when|
|possible. However, some Internet mail constructs such as encoded |
|X.400 addresses (RFC 2156) will often require larger objects. |
|Clients MAY attempt to transmit these, but MUST be prepared for a |
|server to reject them if they cannot be handled by it. To the |
|maximum extent possible, implementation techniques that impose no |
|limits on the length of these objects should be used. Extensions |
|to SMTP may involve the use of characters that occupy more |
|than a single octet each. This section therefore specifies lengths|
|in octets where absolute lengths, rather than character counts, are|
|intended. |
+-------------------------------------------------------------------+
So the 64 character local-part limitation is a "SOFT LIMIT" for
receiving servers. i.e. A receiving server MUST support AT LEAST 64
maximum characters in the local-part.
A receiving server SHOULD remove the local-part 64 character
limitation whenever possible.
A receiving server can explicitly inform the client that it do not
perform the local-part 64 character limitation check with the help of
EAML SMTP extension. EAML SMTP extension is defined in Section 7.
6.4. Clients
The primary motivation for relaxing 64 character local-part
limitation is to have better bounce handling. Both VERP and SRS are
designed to handle bounces.
A bounce message or just "bounce" is an automated message from an
email system, informing the sender that the message had not been
delivered (or some other delivery problem occurred). The original
message is said to have "bounced". More formal terms for bounce
message include "Non-Delivery Report" or "Non-Delivery Receipt"
(NDR), "Delivery Status Notification" (DSN) message, or a "Non-
Delivery Notification" (NDN).
A "bounce address" is also known as MAIL FROM, Return Path, Reverse
Path, Envelope From, Envelope Sender etc.
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As mentioned in [RFC2821] and [RFC5321], a client MAY attempt to
transmit longer email addresses, but MUST be prepared for a server to
reject them if they cannot be handled by it.
6.5. Form Inputs
Form Inputs (e.g. Web registration form) rarely check whether the
local-part contains 64 characters or not. However, there is a small
number of applications certainly do check the local-part character
limit.
For example, Embedded Applications (e.g. software found in a DSLR
camera), cannot upgrade this limit even if we completely remove that
limitation. (We are assuming they already force this 64 character
limit)
As mentioned in the last section, the primary motivation for relaxing
64 character local-part limitation is to have better bounce handling.
i.e. The MAIL FROM address.
Form inputs however, deals with the RCPT TO address. It is very rare
for an end user to have a genuine email address like
this-electronic-mail-address-local-part-contains-62-
characters@example.com
For backward-compatibility reasons, this document doesn't mandate
removing the local-part limitation for end user form inputs.
However, any kind of end user form inputs SHOULD remove the local-
part 64 character limitation WHENEVER POSSIBLE.
7. Framework for the Email Address Maximum Length Extension
This extension is based on the standard SMTP "Service Extensions"
described in [RFC1869].
The following service extension is defined:
(1) The name of the SMTP service extension is "Email Address Maximum
Length";
(2) The EHLO keyword value associated with this extension is "EAML";
(3) One OPTIONAL parameter is allowed with this EHLO keyword value,
a decimal number indicating the maximum email address length in
octets that the server will accept.
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(4) [RFC822] / [RFC2822] / [RFC5322] header field data is still
acceptable to 1000, 998+EOL, where EOL in the SMTP protocol are
the control codes, <CR><LF>, carriage return, linefeed. In order
to avoid hitting that limit and give some room for header keys,
EAML parameter value MUST NOT be greater than 900. i.e. 900
octets is the maximum allowed email address length.
(5) The parameter value SHOULD be from 254-900 (inclusive).
S: 220 smtp.example.com ESMTP Ready
C: EHLO bob.example.org
S: 250-smtp.example.com
S: 250-EXPN
S: 250-EAML 500
S: 250-PIPELINING
S: 250 HELP
(6) The syntax of the parameter is as follows, using the ABNF
notation of [RFC5322] / [RFC2822] / [RFC822]:
eaml-param ::= [3DIGIT]
(7) If the parameter is omitted or the parameter value is 0-253
(inclusive) or the value is greater than 900, then the value
MUST be treated as 254.
(8) Servers offering this extension MUST remove the 64 characters
local-part limit. That effectively means, maximum characters
allowed in local-part is n-2. Where n is the EAML parameter
value.
(9) Servers offering this extension MUST remove the 255 characters
domain-part limit. That effectively means, maximum characters
allowed in domain-part is n-2. Where n is the EAML parameter
value.
(10) The parameter value is OPTIONAL.
S: 220 smtp.example.com ESMTP Ready
C: EHLO bob.example.org
S: 250-smtp.example.com
S: 250-EXPN
S: 250-EAML
S: 250-PIPELINING
S: 250 HELP
(11) The parameter omitted EAML keyword says:
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[*] No local-part limitation.
[*] No domain-part limitation.
[*] 254 maximum characters
(12) No additional SMTP verbs are defined by this extension.
8. IANA Considerations
IANA is hereby requested to register the EAML extension.
9. Security Considerations
This RFC does not discuss security issues and is not believed to
raise any security issues not already endemic in electronic mail and
present in fully conforming implementations of SMTP.
10. References
10.1. Normative References
[RFC821] Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC
821, DOI 10.17487/RFC0821, August 1982, <http://www.rfc-
editor.org/info/rfc821>.
[RFC2821] Klensin, J., Ed., "Simple Mail Transfer Protocol", RFC
2821, DOI 10.17487/RFC2821, April 2001, <http://www.rfc-
editor.org/info/rfc2821>.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
DOI 10.17487/RFC5321, October 2008, <http://www.rfc-
editor.org/info/rfc5321>.
[RFC822] Crocker, D., "STANDARD FOR THE FORMAT OF ARPA INTERNET
TEXT MESSAGES", STD 11, RFC 822, DOI 10.17487/RFC0822,
August 1982, <http://www.rfc-editor.org/info/rfc822>.
[RFC2822] Resnick, P., Ed., "Internet Message Format", RFC 2822, DOI
10.17487/RFC2822, April 2001, <http://www.rfc-
editor.org/info/rfc2822>.
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322, DOI
10.17487/RFC5322, October 2008, <http://www.rfc-
editor.org/info/rfc5322>.
[RFC1869] Klensin, J., Freed, N., Rose, M., Stefferud, E., and D.
Crocker, "SMTP Service Extensions", STD 11, RFC 1869, DOI
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10.17487/RFC1869, November 1995, <http://www.rfc-
editor.org/info/rfc1869>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, DOI
10.17487/RFC2119, March 1997, <http://www.rfc-
editor.org/info/rfc2119>.
10.2. Informative References
[RFC3696] Klensin, J., "Application Techniques for Checking and
Transformation of Names", RFC 3696, DOI 10.17487/RFC3696,
February 2004, <http://www.rfc-editor.org/info/rfc3696>.
[1003] Klensin, J., "Errata 1003", July 2005, <https://www.rfc-
editor.org/errata/eid1003>.
[1690] Sayers, D., "Errata 1690", April 2010, <https://www.rfc-
editor.org/errata/eid1690>.
[VERP] Bernstein, D., "Variable Envelope Return Paths", February
1997, <http://cr.yp.to/proto/verp.txt>.
[SRS] Shevek, "The Sender Rewriting Scheme", June 2004,
<https://www.libsrs2.org/srs/srs.pdf>.
Authors' Addresses
Viruthagiri Thirumavalavan
Dombox, Inc.
EMail: giri@dombox.org
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