I can't believe so many people - particularly women - are being so hard on this guy.
Just telling OP to "man up and deal with it" is no help at all. Everybody has different issues to deal with, many of which may seem trivial to other people, but huge to that person. Callously telling them to "get over themselves" is probably the most unhelpful thing anyone could think of saying. It actually re-affirms the perception to themselves that they have problem without doing anything to help solve it, when they might have been struggling to overcome that issue ineffectively for a long time. It's actually very cruel.
Furthermore, all you people giving "slap in the face" advice, should bear in mind that it is standard practice in couple's therapy to consider that if one person in a relationship has an issue, then the relationship has an issue; i.e. two people are involved. None of you have thought OP's wife is in any way responsible for the fact her husband feels insecure about this; you have all sought to absolve her of any blame at all. This is wrong.
Clearly his wife had not done enough to make him feel secure about the matter.
I seriously doubt that a female posting here on exactly the same subject would receive such harsh treatment - i.e. discovering an email written by her husband during their courting period, bragging to his friends about the fact he had been fucking the shit out of a hot dancer with a fantastic body, lovely tidy pussy, and superb sexual skills, and yet not finding any bragging email about herself. This would be bound to create self esteem issues in anyone.
There are some big issues at play here. Anyone familiar with this site should know that penis size is a HUGE source of concern for a lot of men. Also, the fact that she wrote a bragging email about the marine friend, but no bragging email about OP, even though she was courting him at the time. Then there is the fact that she did not tell him the truth about what she had been doing that weekend, until he found out the truth himself.
OP, I'd be really interested to know what your wife has said about this matter. Does she know how much it has affected you? Why has what she has said so far not allayed your concerns?