Check the following steps:
- Recreate the mail.box
- Check the logs on the CMN Mail Connector server, see if the emails go through the CMN server.
- Check the routing documents
- Check the config document first to see if the mail.box(s) is correct one used.
- Run (Set config SMTPClientDebug=3) from Domino Console to see what errors are being shown
- Make sure you have no Anti-Virus on Domino that could be causing this issue