Help!

About a week ago, Listserv Lite stopped sending mail out to my
subscribers.  The logs show a "550 Relaying is prohibited" message for each
recipient.

Specs: Running on an NT 4.0 server with IIS.  There is another machine on
the network that is an Exchange server, but this problem occurs whether I
try to run the mail through my Exchange server and then straight to the
'net via DNS, or straight to my ISP's mail server.

Which machine/program is generating the error message that shows up in the
logs?  I thought it was coming from UUNet's SMTP server, but now I think
it's internal.  Would Listserv generate this message itself?  If so, why?

By the by, this listserv machine and my Exchange server show evidence of
having been at least probed by folks seeking to exploit that new Code Red
worm virus.  The listserv machine, which is also our DNS server, has lost
some zone files, which I had to reconstruct.  I'm thinking all this stuff
might be related somehow.

I've re-installed Listserv too.

Help me.  I need sleep. :)

Thanks,
Tracey