hopefully not throwing more confusion
Hopefully, I'm not confusing you further with these recommendations...
For starters, why not start sendmail or another MTA and see if it can accept the connection and route the mail to the right user's spool file. If success, then shut it down and test without the daemon. Also, if you look at /var/spool/mail do you see files for the user you're sending them to? Tail the file and see if your mail is appended.