He was specifically asking how to play it from within e-mail, not an app that streams all the different file types, although the one you mentioned does that, its not what this topic is about. AirVideo is rated a bit higher regardless of the app store ratings, just look about the internet and compare, but they still wont play it inside an email unless it is a video compatible with iPhone OS.
I downloaded an app last night for $0.99 that opens wmv videos right in the received email. The app is Azul and it has worked fine so far.
I use 2 apps that work very well for viewing wmv files, OPlayer HD and yxplayer2. I know OPlayer can be gotten as a free test app (paid also available), but I don't remember about yxplayer2. I usually start out with OPlayer, which works about 99% of the time. I switch to yxplayer2 for the others. With these two, I've never been sent a wmv file I can't open. Oh, and make sure, if you get it, that you get the yxplayer2 as it's the one that works best with the iPad. Good luck. Marilyn