Our LISTSERV server is set to use our Exchange 5.5 SP4 server for sending
messages.  Worked great until we had some issues with viruses and spam.

Now, LISTSERV will send messages to internal addresses fine, but addresses
outside of our domain don't ever get delivered.  There has to be a simple
solution, but I can't find it.

The SMTP_forward setting in LISTSERV is set to mail.ourdomain.com like it
always has been.

Exchange is set to accept relay from the LISTSERV server's private IP
address, but nowhere else.

I get no error messages.  The only way we know that a message hasn't been
delivered to the external addresses is when someone calls.

Any ideas where to start?