If it were me... I would hire a PI and figure out what is going on. I would not mention the email but save a copy. He is out till 1am most nights, right? Is he working till 1am or going to socialize with his coworkers?
I just don't know if he will be forthcoming. Not trying to spin it negatively... but if there is some sort of unacceptable behavior going on... would he actually admit to it? I mean when he was cheating in the past he lied about it. Why would he be forthcoming now?
Or if she is interested in him but he not her, would he distance himself from her to make you feel comfortable? I am thinking he would think you are overreacting and not do it.
I personally think after all that has happened a friendship with a woman at this stage would strain the marriage b/c he has not done the work to earn back your trust completely.
Again, if he is cheating again- he will definitely lie. So after all that has happened in the past years I think you need to hire a PI. It is the only way you will REALLY know the truth, IMO.
Good luck! I hope it is nothing.
M38, H37 S3, S7 Together 15 yrs Married 8 yrs Bomb July 2008 Inhouse separation "I hate you" "We are over" (too many times to count) Reconciled Sept 2009 (still worth it)