LF S3 Frost Mage video (Lock Rival Storyline)
 

The main thing I remember about this video was that there was a machinima storyline sprinkled throughout (though it was primarily a PvP video) where him and his Warlock buddy were rivals. I might be confusing this part with another vid, but I think the Lock sends him to/traps him in another realm, and the Mage has to find his way out.

Towards the end of the video, when everything's back to normal, he recieves new mail (I think he's ontop of a building) and walks over to the mailbox. Instead of the normal mail screen opening, a realistic postcard pops up, and on it is a picture of his Undead Male Warlock friend, waving in his S3 Arena gear with the gigantic horns, with a caption like "Greetings from Overpowered Land!" or something like that, resulting in the Mage being pissed. It was hilarious. Does this ring any bells?

I can't believe I even know this... kinda sad but I believe you are referring to Korrak - Hot Streak 2

Reythe, I know you'll probably never see this, but that was it. I've been looking for that video for years. Still was to this day, despite not having played WoW in, well, years either. But god I love that video, and I can't thank you enough for helping me solve that mystery. You are a GOD.

The link, in case anyone cares: http://warcraftmovies.com/movieview.php?id=123735

Hahaha! just glad I could help, I was wondering if you'd ever see it considering I posted a couple of months after.