unroutable mail domain ?

I'm getting this error a lot;

This message was created automatically by mail delivery software.

A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:

someonesname@msn.com
unrouteable mail domain "msn.com"
What is the issue here? I get it for aol.com accounts also.

Help would be greatly appreciated.

 

 

 

 

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