Wow, firstly, congratulations! Secondly, I do subscribe to Pop Photo (I only subscribed as it's just £19 for a year which is much cheaper than most of the UK photo mags) and they do seem to have a lot of reader photos in each issue, so I don't quite know why this wouldn't be a legit thing?
If you still have your doubts, why not call them on a main number (which I'm sure you can find on the net, or I'd be happy to have a look at the editorial section in my latest copy if you want) and ask to speak to the person who emailed you?
I shall look out for your image when it's printed and would be happy to send you the copy (usually arrives a few weeks later than it's published in the US I think as they send by sea rather than airmail)