| Internet-Draft | Core Email A/S | January 2022 |
| Klensin, et al. | Expires 4 August 2022 | [Page] |
Electronic mail is one of the oldest Internet applications that is still in very active use. While the basic protocols and formats for mail transport and message formats have evolved slowly over the years, events and thinking in more recent years have supplemented those core protocols with additional features and suggestions for their use. This Applicability Statement describes the relationship among many of those protocols and provides guidance and makes recommendations for the use of features of the core protocols.¶
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.¶
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In its current form, this draft is a placeholder and beginning of an outline for the Applicability Statement that has been discussed as a complement for proposed revisions of the base protocol specifications for SMTP [RFC5321] (being revised as [I-D.ietf-emailcore-rfc5321bis]) and Internet Message Format [RFC5322] (being revised as [I-D.ietf-emailcore-rfc5322bis]). Among other things, it is expected to capture topics that a potential WG concludes are important but that should not become part of those core documents.¶
That form of a standards track document is appropriate because one of the roles of such a document is to explain the relationship among technical specifications, describe how they are used together, and make statements about what is "required, recommended, or elective".¶
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] and [RFC8174].¶
Over the years since RFC 5321 was published in October 2008, usage of SMTP has evolved, machines and network speeds have increased, and the frequency with which SMTP senders and receivers have to be prepared to deal with systems that are disconnected from the Internet for long periods or that require many hops to reach has decreased. During the same period, the IETF has become much more sensitive to privacy and security issues and the need to be more resistant or robust against spam and other attacks. In addition SMTP (and Message Format) extensions have been introduced that are expected to evolve the Internet's mail system to better accommodate environments in which Basic Latin Script is not the norm.¶
This section describes adjustments that may be appropriate for SMTP under various circumstances and discusses the applicability of other protocols that represent newer work or that are intended to deal with relatively newer issues.¶
If the Domain argument to
the EHLO command does not have an address record in the
DNS that matches the IP address of the client, the SMTP
server may refuse any mail from the client as part of
established anti-abuse practice.
Operational experience has demonstrated that the lack of
a matching address record for the the domain name
argument is at best an indication of a poorly-configured
MTA, and at worst that of an abusive host.¶
The address-literal ABNF
non-terminal is used in various places in
[I-D.ietf-emailcore-rfc5321bis] grammar
however, for SMTP connections over the public internet,
an address-literal
as the argument to EHLO command or the
Domain part of the
Mailbox argument to the MAIL
FROM command is quite likely to result in the message
being rejected as a matter of policy at many sites, since
they are deemed to be signs of at best a misconfigured
server, and at worst either a compromised host or a
server that's intentionally configured to hide its
identity.¶
While addresses in top-level domains (TLDs) are syntactically valid, mail to these addresses has never worked reliably. A handful of country code TLDs have top level MX records but they have never been widely used nor well supported. In 2013 [RFC7085] found 18 TLDs with MX records, which dropped to 17 in 2021 despite many new TLDs having been added.¶
Mail sent to addresses with single label domains has typically expected the address to be an abbreviation to be completed by a search list, so mail to bob@sales would be completed to bob@sales.example.com. This shortcut has led to unfortunate consequnces; in one famous case, in 1991 when the .CS domain was added to the root, mail in computer science departments started to fail as mail to bob@cs was now treated as mail to Czechoslovakia. Hence, for reliable service, mail SHOULD NOT use addreses that contain single label domains.¶
As SMTP has evolved over the years, several extensions have become ubiquitous. As a result, the following extensions MUST be supported by SMTP senders and receivers:¶
Similarly, the following extensions SHOULD be supported by SMTP senders and receivers:¶
This section describes adjustments to the Internet Message Format that may be appropriate under various circumstances.¶
The quoted-string ABNF
non-terminal is used in various places in rfc5322bis
grammar.
While it allows for empty quoted string, such construct is
going to cause interoperability issues when used in certain
header fields.
In particular, use of empty quoted strings is NOT
RECOMMENDED in "received-token" (a component of a Received
header field), "keywords" (a component of a Keywords header
field) and "local-part" (left hand side of email addresses).
Use of empty quoted strings is in particular problematic in
the "local-part".
For example, all of the following email addresses are non
interoperable:¶
"".bar@example.com¶
foo.""@example.net¶
""@example.com¶
Use of empty quoted strings is fine in "display-name".¶
When the work leading to the original version of the MIME specification was completed in 1992 [RFC1341], the intention was that it be kept separate from the specification for basic mail headers in RFC 822 [RFC0822]. That plan was carried forward into RFC 822's successors, [RFC2822] and [RFC5322] and the successors of that original MIME specification including [RFC2045]. The decision to do so was different from the one made for SMTP, for which the core specification was changed to allow for the extension mechanism [RFC1425] which was then incorporated into RFC 5321 and its predecessor [RFC2821].¶
Various uses of MIME have become nearly ubiquitous in contemporary email while others may have fallen into disuse or been repurposed from the intent of their original design.¶
It may be appropriate to make some clear statements about the applicability of MIME and its features.¶
It is fairly clear that there will be things that do not fit into the sections outlined above. As one example, if the IETF wants to say something specific about signatures over headers or what (non-trace) headers may reasonably be altered in transit, that may be more appropriate to other sections than to any of the three suggested above.¶
The Emailcore group arose out of discussions on the ietf-smtp group over changes and additions that should be made to the core email protocols. It was agreed upon that it was time to create a working group that would fix many potential errors and opportunities for misunderstandings within the RFCs.¶
This memo includes no requests to or actions for IANA. The IANA registries associated with the protocol specifications it references are specified in their respective documents.¶
All drafts are required to have a security considerations section and this one eventually will.¶
... To be supplied ...¶
RFC Editor: Please remove this appendix before publication.¶