the outcome would be spoofing - writing messages and miss-attributing them (can you still do that on email?)
The user name is folded in with message so that would be a bit harder do than in email were one just has to change some text.
accidental collisions will of course be rare although the soccer pitch problem
4294967295 /4294967296 * 4294967294 /4294967296 ... (4294967296 - n - 1)/4294967296
should we tack on another char?