Yes it is possible that this email is from your friend. There are certainly times when they are in their main base such as Kandahar that service men and women will have non-classified access to a computer and the internet. The fact that he or she is using a gmail, hotmail or yahoo address makes this more not less probable.
Even so, in my opinion you have every reason to be suspicious and very cautious, especially if something in the email is causing you to have doubts. I suggest the best reply, if you consider one to be appropriate is to open the exchange by asking a simple question, the answer to which only your friend is likely to know; such as "Where and when did we first meet?". Two such questions would be better.
Please realize I am replying to your question in a Canadian context, although I doubt the US context would be much different unless there are extenuating circumstances in this particular case.
