This can happen if the file permissions on the spool files for the emails are incorrect. Run the following to reset them:
chmod 1777 /var/spool/mail chown mail:mail /var/spool/mail chmod 1777 /var/spool/virtual chown mail:mail /var/spool/virtual
and for each domain you have, (eg domain.com for user username)
chmod 770 /var/spool/virtual/domain.com chown username:mail /var/spool/virtual/domain.com