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

Note: This RFC has been updated by RFC 1349, RFC 2181, RFC 5321, RFC 5966, RFC 7766, RFC 9210

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
         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.

Notes:

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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