Exchange 2003, “email account does not exist….”

Need some help. Running W2K3 Standard, GFI MailEssentials v12, Exchange 2003 SP2.

Some external people are getting bounce backs with the following message:

The e-mail account does not exist at the organization this message
was sent to. Check the e-mail address, or contact the recipient
directly to find out the correct address.

So if they are sending to me, and iot gets bounced back, I can send a message to them, they can reply, and then they can send to me. What coul dbe the issue????? It is driving me crazy.