What is the appropriate, rational response... when you find your girlfriend is covertly flirting with some dude over E-mail? Like, one of her co-workers?

Nothing TOO suggestive, mind you (nothing like "Let's go meet up and y'know"), but plenty of wink winks and "Hey, girl"s and flirty banter... and as a dude you know EXACTLY what the dude is trying to fandangle there. Even some talk about setting her up a special Hotmail account (that obviously her and I wouldn't be sharing, or I would supposed to be knowing about) so they could talk further over MSN Messenger. More on the matter... she knows I routinely check her E-mail (and tells me to) because it's the E-mail that we routinely receive pictures from our first child that we put up for adoption 4 years ago there. OK? Since the account is tied to her MySpace, anyone with MySpace knows you get little E-mails from MySpace every time someone sends you a message. So there's plenty of those in there, yes? From her galpals, all of them -- except for this one dude in question. Today I noticed she received two MySpace alerts about this dude sending her messages on MySpace, just some random name and I paid it no thought. Later on the same day... while I'm at work and she's at home, I notice these same message alerts have been skillfully deleted by her. From the trashbin, everything, the whole nine yards... which wouldn't be unusual, except for the fact that she hasn't and doesn't ever delete the message reminders from all of her other friends on MySpace.

You see the picture I'm painting? Nothing specifically cheaterish, nothing even too suggestive... but anyone with half a brain or a modicum of common sense/deductive reasoning can deduce exactly where things more than likely might be heading.

I'm just really wondering what the rational course of action should be. Flip the fuck out? I kind of am. I kind of really am.