RFC 821 section 3.7 states "Whenever domain names are used in SMTP
only the official names are used, the use of nicknames or aliases is
not allowed.". This however changed in RFC 2821 section 3.6 where is
states that CNAME RRs may be used.
Currently Sendmail still rewrites the sender domain. Does this mean
that the default behaviour of Sendmail is not RFC compliant or are
there other RFCs that still limits the use of CNAMEs with MX records
(maybe RFC 1034 section 3.6.2)?
3.6 Domains
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 A RRs (as discussed in section 5) are
permitted, as are CNAME RRs whose targets can be resolved, in turn,
to MX or A RRs.
It says a CNAME can be used, if it can be resolved. So sendmail
resolves it. I don't see any problem there.
Under the language of RFC 821, where using a CNAME was not allowed
at all, it was a kind gesture to try resolving it anyway in order
to deal with a non-standards-compliant host. Strictly speaking
maybe using a CNAME could have been grounds for refusing mail. But
we were all more generous in those days.
Joseph Brennan
Columbia University Information Technology
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