I've noticed that MSN has filtered certain URLs in its messages, such as http://www.mediafire.com/

If a message contains the URL, then it won't be sent.

I have not found any more than this, but I'm sure they have blocked other URLs as well.

Is it possible to prevent messages with these URLs in them from being blocked?

For now I am omitting the http://www part but it's less convenient for someone to paste something in their browser than simply click on it.

Thanks for reading my suggestion and sorry if something like this was already mentioned. I searched for "filter" but found nothing like this.

There appears to be two kinds of filtering done on messages. The first kind is the kind done by your client (and by the server if I recall correctly). This filter is done for security from common worms. Generally there is no way that you can tell these messages have been blocked except by watching your connection to the server.

The second kind of filter seems to be done purely on the server side. When you send a message that gets filtered out the server will reply saying "NAK" and you will get the "message could not be delivered" message.

In either case, if the filtering takes place on the server, there's nothing we can do about it except change the url so that it doesn't match. (URL encoding)