mail coming without MX; how ?

[Jukka Salmi]

Andrew McNamara --> info-cyrus (2007-04-26 16:16:49 +1000):
> >if you check the domain "infoservices.in"  with  dnsstuff.com  you can
> >see no MX for that domain.
> >But still mail is coming at user at infoservices.in.    we are using it for
> >our official purposes and infoservices.in 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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