Adium developers have discovered that Microsoft censors IM messages with certain URLs (eg. ones with .info) in them.
The reason MSN gives for this censorship is that it’s to protect users from exploits that use certain URLs. The problem with this reason is that any URL could be an exploit URL; filtering by keyword just isn’t enough, because the attackers can always invent new filenames. The correct solution is simply to fix the exploits.
MSN mucks up URLs in other ways too. It wasted quite a lot of Karan’s and my time couple of years back when it was converting ftp.prateekrungta.com into ftp://ftp.prateekrungta.com. The program Karan was instructing me to use required the URL to be entered without the ftp:// prefix. Naturally it didn’t work. I can’t remember how we discovered that MSN was changing the instructions but I do remember that it left me with a bitter taste. I’m not sure if this is still true because I haven’t used MSN in ages and like the linked blog post, I recommend switching to Google Talk and convincing your friends to do the same.