Leaving your husband over an e-mail affair, if it is in fact limited to that, is an extreme act. However, you need to first confront him to let him know you are aware of what is going on and do not approve. Next, you need to have a serious and calm conversation with him to learn why? Generally, people do not just simply have affairs unless something in their existing relationship is broken or they have a serious personal problem.
It is likely that your husband may not even consider what he is doing to be wrong -- in other words, an attitude of no sex, no foul. This is not true of course but people delude themselves to justify their actions. You probably will need to involve a marriage counselor to address the underlying problem, whatever it may be. Because you say nothing about the rest of your relationship, it is not possible to speculate on what the problem is or why he is doing this. Unfortunately, even if the affair is strictly a distant online affair, he is likely to progress to to more serious betrayals in the future if the two of you do not fix the problem now.
confront him about it tell him it has to end. Or just leave him and tell him ur not going to sit around with a man who doesnt see how lucky he is to have you.
Bust his chops. Let him know it's not acceptable. Hopefully he won't be on 2 computers ( work & home) with her.
Log onto your husbands email account... read the latest email and reply in the most dickish manner possible pretending to be him. =D this way you get to get all that anger out!
Then, go on and talk to him about it! Affairs are usually result of your partner feeling as though they're missing something in their relationship. Talk it out and see what you can do about it- but don't start yelling and screaming at him about it because that won't get you anywhere.
One word....communication......talk to him about it......otherwise it will lead to more than just e-mail..
Save all the evidence of these e-mails, just in case. It's a good thing to have should things take a turn for the worst. Secondly, I would tell him that it's obvious from these e-mails that your relationship with him is not heading in a healthy direction and that you want to work with him, with the help of professionals if need be, to improve and strengthen it.
I know our first instinct is to either bury or get enraged neither of which have very useful long-term affects. The best thing to do is to try and see if you can face this head on as a couple. At least, that's my $0.02