I have a redhat server with cpanel. Whenever an email gets sent from webmail, or to a forwarder, they get this reject email:
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Subject: Mail delivery failed: returning message to sender


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:

****@sbcglobal.net
unrouteable mail domain "sbcglobal.net"
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This happens with domains that are big or small (gmail, msn, etc). Any clues?