I totally agree with what Sternface said.
It is a myth that only unhappily married people start this kind of thing and, once started, it is very difficult to stop it. The only way is to be honest with the OM and honest with yourself that you are either looking for more than is on offer from the friendship, or reading too much into it.
I've been there too, and I know how quickly these friendships can escalate until it seems like there is nothing you can do to stop them. You can. You are making choices all the time when you invest in a friendship that could possibly turn into an affair if you choose to make it happen.
Exercising your willpower before things have gone too far is much much easier than trying to do it once the friendship/affair has turned into something more emotional or physical.
MarinaAzul says invest more in your marriage and less in e-mailing; and, whilst I would agree in part with that assessment, do be careful that you don't apportion 'blame' to your DH or your marriage if the 'fault' lies within you. By that I mean that, in order to give themselves permission to cheat (whether emotionally or physically) many people start to find fault with their DH/DW, when the 'problem' is actually with the cheater (e.g. possible mid-life crisis, low self-esteem, empty nest). It's easy to blame external factors without looking within to see what has made you react in this way to this man.
I made the mistake of looking for problems within my marriage after my first affair ended, and ended up having a second affair before I realised that the problem was how I viewed myself.
It is, as Marina says, very easy to cross the line, and very easy to justify why you should continue the friendship. Take it from one who knows just how these friendships can turn out, and save yourself from a whole load of pain and heartache by nipping this in the bud now.