"Her choice of email address for the shared account raised alarms on my firewall" That's a pretty serious firewall you have there. Assuming that s/he was not

"Her choice of email address for the shared account raised alarms on my firewall" That's a pretty serious firewall you have there. Assuming that s/he was not using encrypted smtp and imap or pop, then you still have a L7 filter that reads and logs email addresses and alarms on them. Now it is unlikely (to me) that you would have a whitelist of acceptable email addresses with which to alarm. I can think of a few other things you might have done to trigger alarms and all of them are pretty distasteful. So I will conclude you simply violated her civil rights and spied on her. I feel sorrow for you, genuinely, that you have had a relationship problem but I suspect that it would have been easier to find out what was going on in the various old fashioned ways, rather than farting around with IT and being a bit creepy.