This error message show up in the mwg-errors/mwg-core.errors.log file everyday just after midnight. Anybody else seen this?
[2011-02-07 00:00:11.400 -06:00] [NotificationPlugin] [SMTPServerError] Error response from SMTP server: RCPT TO - 501 5.1.3 Invalid address
this looks like the SMTP Server is not accepting the eMail MWG tries to send. A tcpdump would probably show more, are you able to create one? Besides that maybe you can have a look at the SMTP Servers log files.