File transfers seem to be really quirky creatures on all the major IM networks.
If you're having troubles with file tranfers, I'd like to suggest trying
BitWise Chat. Not only can it get around one end or the other having a router or firewall, it can send files to users behind a common firewall and also compresses and encrypts your messages.
Just the other day, a user on a 56k modem told me he was sending files at 12.5k/sec. Not too shabby.
Of course, I'm biased. But that doesn't change the fact that it almost always works even when AIM, Y! and MSN fail.
