I don't know what Photobooth records at, but my guess is that it's simply too big for the iPhone. (The phone only plays up to 1.5 Mbps.)
Open the video in Quicktime and press command-i while it's playing. It should tell you the bit rate.
If that's the case, you'll need to convert the video before you e-mail it.
This is how it should be. You wouldn't want them lowering the quality of photobooth just in case you might e-mail it to an iPhone later.