I am a bit wary of writing my opinion down - I think a lot of heated conversation has come from this post. It seems the underlying question is whether its 'ok' to have a non-sexual, secret friendship with this guy from work.
I do think, deep down that betrayal is more complex than whether you've got your pants off or not - its an emotional connection with a new person who is not your spouse, and it sounds like you've got that, at least in part - here.
Your emails with this man are building a kind of intimacy, temporary or otherwise, based on no responsibility (i.e. you both have plenty of that at home) and if kept secret and non-sexual, with no seeming repercussions or worries attached.
But if you carry on this 'friendship' - what happens to the friendship that underpins your marriage? How do you find the time to laugh with your husband, to relax and look forward to seeing him, rather than being destracted by the hope of an email in your hotmail account?
It sounds like a tough thing to do, but you could simply stop the emails.
You don't have to completely cut this guy dead, but you can put him at arms length. You don't need to confront it, or to 'break up' - how could you when this is all so unofficial and 'light flirting'? But I think you should move away from it, unless you want out of this marriage and this guy is the man for you (which it doesn't sound like he is, nor is he available to be so...)
I hope this doesn't sound harsh. I'm not judging you, just the situation.
best wishes, however things go...