mail coming without MX; how ?

Jukka Salmi j+asg at
Thu Apr 26 03:10:35 EDT 2007

Andrew McNamara --> info-cyrus (2007-04-26 16:16:49 +1000):
> >if you check the domain ""  with  you can
> >see no MX for that domain.
> >But still mail is coming at user at    we are using it for
> >our official purposes and is our official site too.
> >
> >I wounder how mail is still coming with out MX ? could any one kindly
> >explain ?
> Most Mail Transport Agents will fall back to the A record if no MX
> records are found. This precident was set by sendmail, and woe betide
> any implementation that ignores precident, but it would be foolish to
> count on all MTAs behaving this way.

With SMTP to you _can_ count on this behaviour. Quoting RFC 2821:

  5. Address Resolution and Mail Handling
  The lookup first attempts to locate an MX
  record associated with the name. [...] If
  no MX records are found, but an A RR is
  found, the A RR is treated as if it was
  associated with an implicit MX RR, with a
  preference of 0, pointing to that host.

Regards, Jukka

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$ ((RANDOM%6)) || rm -rf ~

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