The smtp logs will contain the error if there is one, as Linda suggested.

If the mail is passing smtp auth and then being passed to the spool then the delivery log will contain information about why the mail is or isnt being delivered.

Thank you for the responses Linda and Jade.  I narrowed it down to the user's signature, but following your suggestions, I checked the delivery log and it told me exactly why.  The messages were being deleted because of a content filter rule.  Some of the keyword entries matched strings of letters in the users signature.  Weirdly, the whole keywords were not present in the signature, just a few of the letters.  Anyways the problem is resolved.  Thanks again.