Sieve Working Group A. Melnikov
Internet-Draft Isode Limited
Intended status: Standards Track B. Leiba
Expires: October 23, 2011 Huawei Technologies
April 21, 2011
Sieve Extension: Externally Stored Lists
draft-ietf-sieve-external-lists-07
Abstract
The Sieve scripting language can be used to implement whitelisting,
blacklisting, personal distribution lists, and other sorts of list
matching. Currently, this requires that all members of such lists be
hardcoded in the script itself. Whenever a member of a list is added
or deleted, the script needs to be updated and possibly uploaded to a
mail server.
This document defines a Sieve extension for accessing externally
stored lists -- lists whose members are stored externally to the
script, such as using LDAP (RFC 4510), ACAP (RFC 2244), CardDAV (work
in progress), or relational databases.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on October 23, 2011.
Copyright Notice
Copyright (c) 2011 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
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(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
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Conventions Used In This Document . . . . . . . . . . . . 3
2. Extlists Extension . . . . . . . . . . . . . . . . . . . . 3
2.1. Capability Identifier . . . . . . . . . . . . . . . . . . 3
2.2. :list Match Type for Supported Tests . . . . . . . . . . . 3
2.3. :list Tagged Argument to the "redirect" Action . . . . . . 4
2.4. Other Uses for External Lists . . . . . . . . . . . . . . 5
2.5. Syntax of an Externally Stored List Name . . . . . . . . . 5
2.6. Test valid_ext_list . . . . . . . . . . . . . . . . . . . 6
2.7. Interaction with ManageSieve . . . . . . . . . . . . . . . 6
2.8. Examples . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.8.1. Example 1 . . . . . . . . . . . . . . . . . . . . . . . . 7
2.8.2. Example 2 . . . . . . . . . . . . . . . . . . . . . . . . 8
2.8.3. Example 3 . . . . . . . . . . . . . . . . . . . . . . . . 8
2.8.4. Example 4 . . . . . . . . . . . . . . . . . . . . . . . . 8
2.8.5. Example 5 . . . . . . . . . . . . . . . . . . . . . . . . 9
3. Security Considerations . . . . . . . . . . . . . . . . . 9
4. IANA Considerations . . . . . . . . . . . . . . . . . . . 11
4.1. Registration of Sieve Extension . . . . . . . . . . . . . 11
4.2. Registration of ManageSieve Capability . . . . . . . . . . 11
4.3. Registration of "ab" URI Scheme . . . . . . . . . . . . . 12
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . 13
6. References . . . . . . . . . . . . . . . . . . . . . . . . 13
6.1. Normative References . . . . . . . . . . . . . . . . . . . 13
6.2. Informative References . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . 15
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1. Introduction
This document specifies an extension to the Sieve language [RFC5228]
for checking membership in an external list or for redirecting
messages to an external list of recipients. An "external list" is a
list whose members are stored externally to the Sieve script, such as
using LDAP [RFC4510], ACAP [RFC2244], CardDAV
[I-D.ietf-vcarddav-carddav], or relational databases.
This extension adds a new match type to apply to supported tests, and
a new tagged argument to the "redirect" action.
1.1. Conventions Used In This Document
Conventions for notations are as in [RFC5228] section 1.1, including
the use of [RFC5234].
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 [RFC2119].
2. Extlists Extension
2.1. Capability Identifier
The capability string associated with the extension defined in this
document is "extlists".
2.2. :list Match Type for Supported Tests
ABNF:
MATCH-TYPE =/ ":list"
; only valid for supported tests
The new ":list" match type changes the interpretation of the "key-
list" parameter (the second parameter) in supported tests. When the
match type is ":list", the key-list becomes a list of names of
externally stored lists. The external lists are queried, perhaps
through a list-specific mechanism, and the test evaluates to "true"
if any of the specified values matches any member of one or more of
the lists.
Comparators are not allowed together with the ":list" match type, so
if both are specified in a test, that MUST result in an error.
Queries done through list-specific mechanisms might have the effect
of built-in comparators; for example, queries to certain lists might
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be case-sensitive, while queries to other lists might be done without
regard to case.
Implementations MUST support the use of :list in "address",
"envelope" and "header" tests. Implementations that include the
Variables extension [RFC5229] MUST also support its use in "string"
tests.
Implementations MAY support other tests but MUST raise an error
(which SHOULD be a compile-time error, but MAY be a runtime error)
when a script uses :list with a test for which it is not supported.
To maintain interoperability, other tests that can be used with :list
SHOULD be documented in a specification that defines a capability
string that can be tested (in a "require" statement, or using ihave
[RFC5463]).
For example, testing 'header ["to", "cc"]' against a list would cause
each "to" and "cc" value, ignoring leading and trailing whitespace,
to be queried. If any value is found to belong to the list, the test
returns "true". If no value belongs to the list, the test returns
"false". Once a value is found in the list, there is no need for the
query mechanism to look further.
For some lists, the Sieve engine might directly retrieve the list and
make its own comparison. Other lists might not work that way -- they
might provide a way to ask if a value is in the list, but not permit
retrieval of the list itself. It is up to the Sieve implementation
to understand how to interact with any supported list. If the Sieve
engine is permanently unable to query the list (perhaps because the
list doesn't support the required operation), the test MUST result in
a runtime error in the Sieve script.
See Section 2.5 for the detailed description of syntax used for
naming externally stored lists.
The :list match type uses the concept of "match variables" as defined
in Section 3.2 of the Variables extension [RFC5229]. Implementations
that also support that extension MUST set the ${0} match variable to
the value in the list that matched the query. Other numbered match
variables (${1}, ${2}, and so on) MAY be set with list-specific
information that might be of use to the script.
2.3. :list Tagged Argument to the "redirect" Action
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Usage: redirect :list <ext-list-name: string>
The "redirect" action with the ":list" argument is used to send the
message to the set of email addresses in the externally stored list
named by the ext-list-name string. This variant of the redirect
command can be used to implement a personal distribution list.
For this feature to work, one of the following conditions has to be
true:
1. The list resolves to a list of email addresses, and the Sieve
engine is able to enumerate those addresses.
2. The list handler is able to take care of the redirection on
behalf of the Sieve engine.
In cases where, for example, a list contains hashed email address
values or an email address pattern ("sz*@example.com",
"*+ietf@example.net"), the Sieve engine will not be able to redirect
to that list, and responsibility must pass to the list handler.
If neither the Sieve engine nor the list handler can enumerate (or
iterate) the list, or the list does not resolve to email addresses,
the situation MUST result in a runtime error in the Sieve script.
See Section 2.5 for the detailed description of syntax used for
naming externally stored lists.
2.4. Other Uses for External Lists
The uses for external lists specified here represent the useful cases
and situations at the time of this writing. Other uses for external
lists, using other Sieve features, might be devised in the future,
and such uses can be described in extensions to this document.
2.5. Syntax of an Externally Stored List Name
A name of an externally stored list is always an absolute URI
[RFC3986]. Implementations might find URIs such as LDAP [RFC4510],
CardDAV [I-D.ietf-vcarddav-carddav], or Tag [RFC4151] to be useful
for naming external lists.
The "tag" URI scheme [RFC4151] can be used to represent opaque, but
user friendlier identifiers. Resolution of such identifiers is going
to be implementation specific and it can help in hiding the
complexity of an implementation from end users. For example, an
implementation can provide a web interface for managing lists of
users stored in LDAP. Requiring users to know generic LDAP URI
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syntax might not be very practical, due to its complexity. An
implementation can instead use a fixed tag URI prefix such as "tag:
example.com,<date>:" (where <date> can be, for example, a date
generated once on installation of the web interface and left
untouched upon upgrades) and the prefix doesn't even need to be shown
to end users.
The "ab" URI scheme (in particular, the URI "ab:default"), defined in
Section 4.3 MUST be supported. The mandatory-to-implement URI "ab:
default" gives access to the user's default address book (usually the
user's personal address book).
It's possible that a server will have no access to anything
resembling an address book (perhaps in an implementation where
address books are only client-side things), but the server can still
provide access to other sorts of lists -- consider the list of dates
in Example 2 (Section 2.8.2), or lists of important keywords and the
like. It might sometimes make sense to map "ab:default" into some
available list, but that might not always be reasonable. If there
really is no concept of an address book in a particular server
implementation, the server MAY support "ab:default" by having all
matches to it fail. Such an implementation SHOULD NOT be done except
as a last resort.
Queries against address books SHOULD be done without regard to case.
2.6. Test valid_ext_list
Usage: valid_ext_list <ext-list-names: string-list>
The "valid_ext_list" test is true if all of the external list names
in the ext-list-names argument are supported, and they are valid both
syntactically (including URI parameters) and semantically (including
implementation-specific semantic restrictions). Otherwise the test
returns false.
This test MUST perform exactly the same validation of an external
list name as would be performed by the "header :list" test.
2.7. Interaction with ManageSieve
This extension defines the following new capability for ManageSieve
(see [RFC5804] section 1.7):
EXTLISTS - A space-separated list of URI schema parts [RFC3986] for
supported externally stored list types. This capability MUST be
returned if the corresponding Sieve implementation supports the
"extlists" extension defined in this document.
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This also extends the ManageSieve ABNF as follows:
single-capability =/ DQUOTE "EXTLISTS" DQUOTE SP ext-list-types CRLF
; single-capability is defined in [RFC5804]
ext-list-types = string
; space separated list of URI schema parts
; for supported externally stored list types.
; MUST NOT be empty.
2.8. Examples
2.8.1. Example 1
This example uses a personal address book, along with the Spamtest
[RFC5235] and Relational [RFC5231] extensions to give a different
level of spam tolerance to known senders.
require ["envelope", "extlists", "fileinto", "spamtest",
"relational", "comparator-i;ascii-numeric"];
if envelope :list "from" "ab:default"
{ /* Known: allow high spam score */
if spamtest :value "ge" :comparator "i;ascii-numeric" "8"
{
fileinto "spam";
}
}
elsif spamtest :value "ge" :comparator "i;ascii-numeric" "3"
{ /* Unknown: less tolerance in spam score */
fileinto "spam";
}
The same example can also be written another way, if the Variables
extension [RFC5229] is also supported:
require ["envelope", "extlists", "fileinto", "spamtest",
"variables", "relational", "comparator-i;ascii-numeric"];
if envelope :list "from" "ab:default" {
set "lim" "8"; /* Known: allow high spam score */
} else {
set "lim" "3"; /* Unknown: less tolerance in spam score */
}
if spamtest :value "ge" :comparator "i;ascii-numeric" "${lim}" {
fileinto "spam";
}
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2.8.2. Example 2
This example uses the "currentdate" test [RFC5260] and a list
containing the dates of local holidays. If today is a holiday, the
script will notify [RFC5435] the user via XMPP [RFC5437] about the
message.
require ["extlists", "date", "enotify"];
if currentdate :list "date"
"tag:example.com,2011-01-01:localHolidays" {
notify "xmpp:romeo@im.example.com";
}
2.8.3. Example 3
This example also uses the "envelope" option [RFC5228] and the
Subaddress extension [RFC5233]. If mail is sent with the list name
as a subaddress of the recipient (to, say, "alexey+mylist"), and the
message comes from a member of the list, it will be redirected to all
members of the list. Variants of this technique might be useful for
creating private mailing lists.
require ["extlists", "envelope", "subaddress"];
# Submission from list members is sent to all members
if allof (envelope :detail "to" "mylist",
header :list "from"
"tag:example.com,2010-05-28:mylist") {
redirect :list "tag:example.com,2010-05-28:mylist";
}
2.8.4. Example 4
This example uses variable matching [RFC5229] to extract the IP
address from the last "Received" header field. It then checks that
against a "block list" of undesirable IP addresses, and rejects the
message if there's a match.
require ["variables", "extlists", "index", "reject"];
if header :index 1 :matches "received" "*(* [*.*.*.*])*" {
set "ip" "${3}.${4}.${5}.${6}";
if string :list "${ip}"
"tag:example.com,2011-04-10:DisallowedIPs" {
reject "Message not allowed from this IP address";
}
}
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2.8.5. Example 5
This example uses several features of the MIME parts extension
[RFC5703] to scan for unsafe attachment types. To make it easily
extensible, the unsafe types are kept in an external list, which
would be shared among all users and all scripts, avoiding the need to
change scripts when the list changes.
[Note that this is an illustrative example, and more rigorous malware
filtering is advisable. It is insufficient to base email security on
checks of filenames alone.]
require [ "extlists", "foreverypart", "mime", "enclose" ];
foreverypart
{
if header :mime :param "filename"
:list ["Content-Type", "Content-Disposition"]
"tag:example.com,2011-04-10:BadFileNameExts"
{
# these attachment types are executable
enclose :subject "Warning" :text
WARNING! The enclosed message attachments that might be unsafe.
These attachment types may contain a computer virus program
that can infect your computer and potentially damage your data.
Before clicking on these message attachments, you should verify
with the sender that this message was sent intentionally, and
that the attachments are safe to open.
.
;
break;
}
}
3. Security Considerations
Security considerations related to the "address"/"envelope"/"header"
tests and "redirect" action discussed in Sieve [RFC5228] also apply
to this document.
External list memberships ought to be treated as if they are an
integral part of the script, so a temporary failure to access an
external list SHOULD be handled in the same way as a temporary
failure to retrieve the Sieve script itself.
For example, if the Sieve script is stored in the Lightweight
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Directory Access Protocol [RFC4510] and the script can't be retrieved
when a message is processed (perhaps the LDAP server is unavailable),
then the Sieve engine might delay message delivery until the script
can be retrieved successfully. Similarly, if an external list is
stored in LDAP and that LDAP server is unavailable, the Sieve engine
would take the same action -- delay message delivery and try again
later.
Protocols/APIs used to retrieve/verify external list membership MUST
provide an appropriate level of confidentiality and authentication.
Usually, that will be at least the same level of confidentiality as
protocols/APIs used to retrieve Sieve scripts, but only the
implementation (or deployment) will know what is appropriate.
There's a difference, for example, between making an LDAP request on
a closed LAN that's only used for trusted servers (it may be that
neither encryption nor authentication is needed), on a firewalled LAN
internal to a company (it might be OK to skip encryption, depending
upon policy), and on the open Internet (encryption and authentication
are probably both required). It also matters whether the list being
accessed is private or public (no encryption or authentication may be
needed for public data, even on the Internet).
Implementations of this extension should keep in mind that matching
values against an externally stored list can be IO and/or CPU
intensive. This can be used to deny service to the mailserver and/or
to servers providing access to externally stored mailing lists. A
naive implementation, such as the one that tries to retrieve content
of the whole list to perform matching can make this worse.
But note that many protocols that can be used for accessing
externally stored lists support flexible searching features that can
be used to minimize network traffic and load on the directory
service. For example, LDAP allows for search filters.
Implementations SHOULD use such features whenever they can.
Many organizations support external lists with thousands of
recipients. In order to avoid mailbombs when redirecting a message
to an externally stored list, implementations SHOULD enforce limits
on the number of recipients and/or on domains to which such
recipients belong.
Note in particular that it can be too easy for a script to use
redirect :list "ab:default";
to send messages to "everyone in your address book", and one can
easily imagine both intentional and accidental abuse. The situation
can be even worse for, say, "ab:corporate". Warnings, as well as
enforced limits, are appropriate here.
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4. IANA Considerations
4.1. Registration of Sieve Extension
The following template specifies the IANA registration of the Sieve
extension specified in this document. This information should be
added to the list of sieve extensions given on
http://www.iana.org/assignments/sieve-extensions.
To: iana@iana.org
Subject: Registration of new Sieve extension
Capability name: extlists
Description: Adds the ":list" match type to certain Sieve tests, and
the ":list" argument to the "redirect" action. The ":list" match
type changes tests to match values against values stored in one
or more externally stored lists. The ":list" argument to the
redirect action changes the redirect action to forward the
message to email addresses stored in the externally stored list.
RFC number: this RFC
Contact address: Sieve mailing list <sieve@ietf.org>
4.2. Registration of ManageSieve Capability
The following requests IANA to register a new ManageSieve Capability
according to the IANA registration template specified in [RFC5804]:
To: iana@iana.org
Subject: ManageSieve Capability Registration
Capability name: extlists
Description: This capability is returned if the server supports the
"extlists" [RFCXXXX] Sieve extension.
Relevant publications: this RFC, Section 2.7
Person & email address to contact for further information: Sieve
mailing list <sieve@ietf.org>
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Author/Change controller: IESG
4.3. Registration of "ab" URI Scheme
The following requests IANA to register a new URI scheme according to
the IANA registration template specified in [RFC4395]:
URI scheme name: ab
Status: Permanent
URI scheme syntax:
paburi = "ab:" addrbook [ "?" extensions ]
addrbook = segment
; <segment> defined in [RFC3986]
extensions = query
; <query> defined in [RFC3986]
URI scheme semantics: "ab" URIs are used for designating references
to address books. An address book is an internal concept used by
different applications (such as Sieve interpreters) for
describing a list of named entries, and may be translated into
other types of address books, such as LDAP Groups. Address books
may be private or shared; they may be personal, organizational,
or perhaps even "crowdsourced".
Encoding considerations: Percent-encoding is allowed in "segment"
and "query" components. Internationalization is handled by IRI
processing.
Intended usage: An "ab" URI is designed to be used internally by
applications for referencing address books. Each URI is intended
to represent a grouping of addresses that can be logically
thought of as one "book". Any given address can belong to more
than one book -- that is, can be referred to by more than one
URI.
The URI "ab:default" is a reserved name that MUST be implemented,
representing a default grouping (book) of addresses. Other
names, representing the same or other groupings MAY be
implemented. For example, an implementation might have the
following URIs:
* ab:personal -- a book representing the user's personal
address book.
* ab:friends -- a subset of ab:personal, defined by the user.
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* ab:family -- a subset of ab:personal, defined by the user.
* ab:company -- a book representing user's company's address
book.
* ab:department -- a subset of ab:company, defined by the
company.
* ab:co-workers -- a subset of ab:company, defined by the user.
* ab:default -- the default address book, a reference to ab:
personal.
Applications and/or protocols that use this URI scheme name:
Currently only the Sieve External List extension is using this
URI scheme. Email clients that use URIs internally might find
this URI scheme to be useful as well.
Interoperability considerations: Applications are only REQUIRED to
support "ab:default".
Security considerations: Applications SHOULD ensure appropriate
restrictions are in place to protect sensitive information that
might be revealed by "ab" URIs from access or modification by
untrusted sources.
Relevant publications: this RFC
Contact: Sieve mailing list <sieve@ietf.org>
Author/Change controller: IETF/IESG
5. Acknowledgements
Thanks to Alexandros Vellis, Nigel Swinson, Ned Freed, Kjetil Torgrim
Homme, Dave Cridland, Cyrus Daboo, Pete Resnick, and Robert Burrell
Donkin for ideas, comments and suggestions. Kristin Hubner also
helped greatly with the examples.
6. References
6.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
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[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
[RFC4151] Kindberg, T. and S. Hawke, "The 'tag' URI Scheme",
RFC 4151, October 2005.
[RFC4395] Hansen, T., Hardie, T., and L. Masinter, "Guidelines and
Registration Procedures for New URI Schemes", BCP 35,
RFC 4395, February 2006.
[RFC5228] Guenther, P. and T. Showalter, "Sieve: An Email Filtering
Language", RFC 5228, January 2008.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC5804] Melnikov, A. and T. Martin, "A Protocol for Remotely
Managing Sieve Scripts", RFC 5804, July 2010.
6.2. Informative References
[I-D.ietf-vcarddav-carddav]
Daboo, C., "vCard Extensions to WebDAV (CardDAV)",
draft-ietf-vcarddav-carddav-10 (work in progress),
November 2009.
[RFC2244] Newman, C. and J. Myers, "ACAP -- Application
Configuration Access Protocol", RFC 2244, November 1997.
[RFC4510] Zeilenga, K., "Lightweight Directory Access Protocol
(LDAP): Technical Specification Road Map", RFC 4510,
June 2006.
[RFC5229] Homme, K., "Sieve Email Filtering: Variables Extension",
RFC 5229, January 2008.
[RFC5231] Segmuller, W. and B. Leiba, "Sieve Email Filtering:
Relational Extension", RFC 5231, January 2008.
[RFC5233] Murchison, K., "Sieve Email Filtering: Subaddress
Extension", RFC 5233, January 2008.
[RFC5235] Daboo, C., "Sieve Email Filtering: Spamtest and Virustest
Extensions", RFC 5235, January 2008.
[RFC5260] Freed, N., "Sieve Email Filtering: Date and Index
Extensions", RFC 5260, July 2008.
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[RFC5435] Melnikov, A., Leiba, B., Segmuller, W., and T. Martin,
"Sieve Email Filtering: Extension for Notifications",
RFC 5435, January 2009.
[RFC5437] Saint-Andre, P. and A. Melnikov, "Sieve Notification
Mechanism: Extensible Messaging and Presence Protocol
(XMPP)", RFC 5437, January 2009.
[RFC5463] Freed, N., "Sieve Email Filtering: Ihave Extension",
RFC 5463, March 2009.
[RFC5703] Hansen, T. and C. Daboo, "Sieve Email Filtering: MIME Part
Tests, Iteration, Extraction, Replacement, and Enclosure",
RFC 5703, October 2009.
Authors' Addresses
Alexey Melnikov
Isode Limited
5 Castle Business Village
36 Station Road
Hampton, Middlesex TW12 2BX
UK
Email: Alexey.Melnikov@isode.com
Barry Leiba
Huawei Technologies
Phone: +1 646 827 0648
Email: barryleiba@computer.org
URI: http://internetmessagingtechnology.org/
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