Thanks for your quick replies!

No, not sending from any free email. We have our own GroupWise mail server.
But it has nothing to do with that either, since I'm able to replicate the problem making a direct connection to the Hotmail server with a Telnet session, like I described in my first message.

As far as I can tell, Hotmail are the only ones we get undeliverable email notifictaions...

Really appreciate your help!
Any other ideas?