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RFC 1123, "Requirements for Internet Hosts - Application and Support", October 1989

Source of RFC: Legacy

Errata ID: 6134
Status: Rejected
Type: Technical
Publication Format(s) : TEXT

Reported By: Abel
Date Reported: 2020-04-28
Rejected by: Barry Leiba
Date Rejected: 2020-04-28

Section 5.2.2 says:

5.2.2  Canonicalization: RFC-821 Section 3.1

         The domain names that a Sender-SMTP sends in MAIL and RCPT
         commands MUST have been  "canonicalized," i.e., they must be
         fully-qualified principal names or domain literals, not
         nicknames or domain abbreviations.  A canonicalized name either
         identifies a host directly or is an MX name; it cannot be a

It should say:

RFC5321 Section 2.3.5.  Domain Names

Only resolvable, fully-qualified domain names (FQDNs) are permitted
   when domain names are used in SMTP.  In other words, names that can
   be resolved to MX RRs or address (i.e., A or AAAA) RRs (as discussed
   in Section 5) are permitted, as are CNAME RRs whose targets can be
   resolved, in turn, to MX or address RRs.


Section 2.3.5 from RFC 5321 seems to contradict the section 5.2.2 from 1123.

In RFC5321 it is implied that you can have a domain CNAME pointing to another that has an MX record that is not a CNAME and it should work. However 1123 states that it's not possible.
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