Log and monitor all that you can. And watch for a large number of IPs logging into an account over a day (over a set limit - even across country - that takes into account "home - blackberry - airport lounge - airport lounge in another country - hotel - RIPE meeting venue" type scenarios).
And especially watch for and/or firewall off logins from areas from where you see particularly high levels of smtp auth abuse / logins to compromised accounts --srs 2010/4/21 Alex Kamiru <nderitua...@gmail.com>: >>>Inside customers, we have not changed to force port 587 and >>>authentication for email clients, but the topic has come up in >>>discussions. This won't of course, stop spammers if they are hijacking >>>the users local email client settings. > > How best would you stop spammers hijacking local users email clients > > -Mike