[svlug] About exim bounce messages

[Jeffrey Siegal]

"Derek J. Balling" wrote:
> You believe wrong. *grin* It is only required if you wish to
> designate "some other site to act as a Mail eXchanger".  If
> $RIGHT_HAND_SIDE doesn't have an MX, you'd damned well better try
> sending it direct to the A record, so sayeth the relevant RFC's.

Absolutely correct.

 RFC 974 January 1986
 Mail Routing and the Domain System

 It is possible that the list of MXs in the response to the query will
 be empty.  This is a special case.  If the list is empty, mailers
 should treat it as if it contained one RR, an MX RR with a preference
 value of 0, and a host name of REMOTE.  (I.e., REMOTE is its only
 MX).  In addition, the mailer should do no further processing on the
 list, but should attempt to deliver the message to REMOTE.  The idea
 here is that if a domain fails to advertise any information about a
 particular name we will give it the benefit of the doubt and attempt
 delivery.

["REMOTE" here refers to the mail destination for which DNS is being
queried.]

It is generally considered good form to have an MX record for any domain
that is going to receive mail, because otherwise the sender will have to
look for one and have it fail before looking up the A record.  If the MX
record exists, it can be cached.  (A record that doesn't exist can only
be negatively cached, which is less efficient.)