[Exim] Returned message

I'm not sure if this is an Exim related question. Can anyone assist with
this error message:

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. The following address(es) failed:

  charles@???:
    retry time not reached for any host after a long failure period

I can mail this person using a different mail server program, but when
mailed through Exim, it returns this message.

What gives??

Thanks