missing email messages

I have an issue with incoming email messages from a single domain.  My client uses a fax service which converts the fax to an email message and then sends it to them.  In the past these messages would come in and the client could retrieve them from the mailbox.  Recently they have stopped coming in, or at least that is what we thought.  The first thing I checked was to have the fax service send a test email from a user of the same domain as the fax service.  That test email message never made it, or so we thought. After I checked the agent log of the exchange server I can see the message is accepted by our exchange server but I have no idea where it is going after that.  I was hoping there is a command or utility I can use on the clients server to get a some detail on the delivery of these messages.  I have checked local PC's for any rules which may be directing the message somewhere else.  I have also tried adding the domain and the email address of the fax service to the whitelist on the exchange server and still no messages are making it to the fax mailbox.  Any suggestions would be greatly appreciated.