troubleshooting Question

exchange server stopped sending out mail.recipient(s) could not be reached: The e-mail address could not be found...#5.1.0 smtp;550 5.1.0 Authentication req

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gtdideaFlag for United States of America asked on
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As of Friday afternoon the SMTP server just stopped sending out email.  emails can be received but not sent.  I am at a loss as i checked all settings, check with ISP made sure it was not on their end  as well as the domain host.  Don't know what else to do.

Your message did not reach some or all of the intended recipients.

Subject: RE: test
Sent: 5/16/2011 2:39 PM

The following recipient(s) could not be reached:

  Guy Dunn on 5/16/2011 2:39 PM
  The e-mail address could not be found. Perhaps the recipient moved to a different e-mail organization, or there was a mistake in the address. Check the address and try again.
  < #5.1.0 smtp;550 5.1.0 Authentication required>
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