domain doesn't resolve (but hosts do)

[glen herrmannsfeldt]

Only an MX record is required for receiving mail, and this should not
change.  There must be an A for the destination of the MX, though.
(The destination might be through a uucp link, or otherwise not directly
reachable.)

Your question is the from address, which is different.  I believe,
that because of spam, smtp servers are checking the source address
more carefully, and that is the problem as you describe.  Note that 
the sending machine does not need to run an smtp server.  The MX for
that name could point to a different machine.  The recipient just wants
to verify that your mail header isn't fake, and so you need an A.
It might also verify the reverse, so you should be sure to have a PTR, also.

-- glen

woods at ncar.UCAR.EDU (Greg Woods) writes:

>We are having a problem here that sounds similar to the folks that
>haven't paid their NSI bills, but I do not believe that is our problem.
>The problem is that mail being sent with an SMTP sender ending with
>"@ucar.edu" (our domain name) is being rejected with an unresolveable
>domain error. Mail sent from other hosts here, e.g. "@host.ucar.edu",
>is working fine. (one example is infi.net, which is still rejecting our
>mail, but whose name servers don't seem to have a problem resolving our
>domain name). I cannot find anything wrong with our primary or
>secondary DNS servers. The information for this particular DNS record
>has not been changed in over a year. The domain has SOA, NS, and MX
>records, but no A record (my reading of RFC1123 says that an A record
>is not required, and we have never had one). This problem just started
>Thursday afternoon (Sept. 9) and is still continuing, although some
>domains that were unable to send us mail (such as earthlink.net) are
>now able to.