spam filter and MX records

[Barry Margolin]

In article <drlk1k$teo$1 at sf1.isc.org>,
 "barber.greg at gmail.com" <barber.greg at gmail.com> wrote:

> We've recently implemented a spam filter for our webmail server. Prior
> to putting the filter in place all mail was sent directly to the server
> via an A  record in DNS. The A record was also the domain name for the
> mail server so you had someting like this user at xyz.123.edu recived mail
> 
> at
> xyz.123.edu IN  A  192.xxx.xxx.xxx .
> 
> For the filter to work properly an MX record was created to point to
> the filter like this
> 
> xyz.123.edu IN MX mail.filter.123.edu
> 
> I suggested that the A record also be change, during testing I noticed
> messages being delivered straight to the host and not the by the MX
> designation.  Would it be safe to change the A record back to
> xyz.123.edu? I've read some mailers don't respect MX records and
> deliver to the host anyway. Besides isn't bad form to have your A
> record the same as your mail domain?

I've heard the opposite -- that there are some mailers that only look 
for MX records.  Also, some spamming programs work like this, so I've 
heard people advocate not use MX records as a way to reduce the amount 
of spam that's received.

Any mailer that doesn't use MX records is horribly broken, and I don't 
know why you'd care whether they can send mail to you.  MX records have 
been part of the mail standard for 20 years, it makes no sense to me 
that someone would implement an MTU that doesn't use them.  Any site 
using that mailer probably wouldn't be able to send mail to users at 
aol.com, yahoo.com, or hotmail.com, since their A records all point to 
the web site rather than their mail servers.

-- 
Barry Margolin, barmar at alum.mit.edu
Arlington, MA
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