Here's something I hadn't seen before: I IM'd a friend of mine at a Big Company and the message got rejected because it included a URL. I have to suppose that it's any URL that gets bounced, but who knows?
So the workaround? I sent the text for a Google query that returns the same url.
Further details: Discussing it with him, the logic seems to be that someone could send an employee a phishing link in an IM message.