Lately messages I have sent have been bouncing back with the error message:  
550 host unkown.  I am sending to valid addresses, so I don't know why they
aren't working.

I recently created a distribution list with about 30 addresses - that's when
the problem started occuring.  Could it related?  

I contacted my ISP but they claim it is an Outlook problem.

Thanks for any help you can offer.