Incoming Email Problems (550 5.7.1. relaying denied)

Good morning,

I'm having problems with incoming email to my network. Mail coming to
my exchange 2003 server seems to get stopped somewhere, but I'm having
trouble tracking it down.

I discovered the problem when users on my network were being removed
from mailing lists due to undeliverables. I recently found out that
they are later getting notifications of this saying "550 5.7.1.
<user@xxxxxxxxxxxxx>...Relaying Denied."

Its not ALL incoming email thats getting stopped, but mostly
newsletters, etc. I've also noticed I'm not getting my online bank
statements due to undeliverable errors they are recieving.

Can anyone help with this? If you need more information, I'd be glad
to provide it. Just let me know what you need to know.