RFC 1123, "Requirements for Internet Hosts - Application and Support", October 1989Source of RFC: Legacy
Errata ID: 6134
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 CNAME.
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.
Errata reports are for mistakes in the published document -- errata at the time of publication. Anything that might have changed since then is appropriate for a new document, but isn't errata in the old one.