Uncle Google is thwarting me on this one ...

If I send a short SMS text message to multiple recipients from my iPhone, the people at the other end, if they don't have an iPhone themselves, will receive some kind of weird multimedia download message. On one phone I saw, the message had an animated border around it, for example. On another, the recipient was warned of a 2k download, got a total of three messages, and finally received my text in the third message.

What gives here? This doesn't happen with a single-recipient message.