Originally Posted by *LostMyWay*
If you and your gf have complete trust then theres no reason you can't tell her about it. Tell the girl that you are taken already, will be easier for her that way. Be honest with your gf about the email (possibly neglect to tell her who its from straight away, if she asks then tell her) and reassure her that you weren't tempted or interested, that you only have eyes for her etc etc.
That way you don't actually lie to either of them, you don't cheat on either of them and your gf will trust you all the more for opening up to her.
I'm not saying all the other advice wasn't good, but that was...... really good. Thank you all! I did the above. It worked out, and she did ask who it was from, and she seemed okay. Thnaks again. lol