Okay, here's a general question for the group at large:
You are doing an MX request to resolve an address that
you need to send to. You get back a Server Error (RCODE=2)
or an illegal return code (RCODE=6..15). What is the best
thing to do to try and resolve this?
You have to abort the delivery attempt and try again later (temporary error).
No other course of action is acceptable, really -- there is nothing that says
that trying the host directly (even assuming you can get its A record when you
can't get its MX record list) will produce correct results. Even if you're
willing to enter a special mode where you treat any errors you get under these
circumstances as temporary, it won't work in general. There are cases where the
host in user(_at_)host being an MX pointing to several other systems ends up
resolving to an entirely different mailbox than user(_at_)host where host is
treated
as simply an A record. There are also cases where direct delivery ignoring MX
leads to loops. (Yes, such setups are problematic given the fact that not
everything on the Internet is consistent in its support of MX, but such setups
exist nevertheless and have to be dealt with.)
The bottom line is that RFC974 says that when an the DNS returns a non-empty MX
list the client is supposed to try all the hosts on that list and then stop,
and setups have been built and are actively being used that take advantage of
every nuance of this described behavior. So if you cannot get the MX
list you don't know what you should or should not do, so your only
course of action is to not do anything.
Ned