mail coming without MX; how ?

[Bron Gondwana]

On Thu, Apr 26, 2007 at 09:10:35AM +0200, Jukka Salmi wrote:
> 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.

The difference between theory and practice: in theory they are the same.

With SMTP you can't rely on <insert rude words here> anything.  For every
rule there's a broken application that doesn't follow it.  The only
question is how liberal you're willing to be in what you accept and how
careful you're going to be in what you send to get the level of inter-
operability that your users will appreciate.

Bron.