In message <Pine.A32.3.96.980706111455.15138G-100000@macro.stanford.edu>, Bill 
Studenmund writes:
>  This message is in MIME format.  The first part should be readable text,
>  while the remaining parts are likely unreadable without MIME-aware tools.
>  Send mail to mime@docserver.cac.washington.edu for more info.

Hmph.

>... while talking to mail.weird.com.:
>>>> MAIL From:<skippy@macro.stanford.edu> SIZE=2829
><<< 550 '<skippy@macro.stanford.edu>SIZE=2829' sender address target
>'macro.stanford.edu' is not a valid e-mail domain (there is no MX record
>for it). 

This is probably a spam filter.  The idea is to reject "rapid fire mail
server" style messages, which are sent directly from dialups, and only
accept mail from machines which appear to be mail exchangers.  I believe
this is misguided.

>??? What is this poop? Since when do hosts HAVE to have MX records?

It's a common bug for some MTA's to be confused and unable to talk to a
host without an MX record for it.

>macro.stanford.edu most CERTAINLY has an "A" record. Why isn't that good
>enough?

Probably because the filter is a bit stricter?

-s