Mail in Ubuntu. Sendmail log file says 'User unknown'

I was getting the error'

I realized it was because my domain name was set in the local-host-names file.

To fix this I:

1. opened up the 'local-host-names' file
i.e.
sudo nano /etc/mail/local-host-names

2. Removed my domain name from the file.

3. Then restarted sendmail:

/etc/init.d/sendmail restart