[Mimedefang] Domain canonifyin​g and RFCs

[Ernst du Plooy]

I posted the following on the sendmail (comp.mail.sendmail) newsgroup,
but I don't get any reaction.  I realise this is not a mimedefang
problem, but I'm sure some of you have also run into this issue.

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)?

Thanks

P.S.  I know the setting confDONT_EXPAND_CNAMES will disable this
behaviour, but I would prefer not to implement this.  Also I cannot
modify the DNS records since it is an external DNS server.
They insist their DNS settings are RFC compliant.

Their DNS records looks like this:
c.domain.com      IN      A       1.1.1.1
a.b.domain.com.  IN      CNAME   c.domain.com.
c.domain.com.    IN      MX      10   mail.domain2.com
c.domain.com.    IN      MX      20   mail2.domain2.com

user at a.b.domain.com will be rewitten to user at c.domain.com