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From:  Mail Delivery Subsystem   <mailer-daemon@googlemail.com>
Date: Mon, Mar 14, 2016 at 9:22 AM
Subject: Delivery Status Notification (Failure)
To:  XYZ@gmail.com


Delivery to the following recipient failed permanently:

      tsalem@XYZ.net

Technical details of permanent failure:
Google tried to deliver your message, but it was rejected by the relay  smtp.zoho.com  [165.254.168.48].

The error that the other server returned was:
535 Authentication Failed
 (SMTP AUTH failed with the remote server)

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I was also got this error message:

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The following message could not be forwarded to  XYZ@gmail.com  for  dsalem@XYZ.net

Technical details of the failure:
4.7.0 [74.201.84.158      15] Our system has detected an unusual rate of
                        4.7.0 unsolicited mail originating from your IP address. To protect our
                        4.7.0 users from spam, mail sent from your IP address has been temporarily
                        4.7.0 rate limited. Please visit
                        4.7.0   https://support.google.com/mail/answer/81126  to review our Bulk Email
                        4.7.0 Senders Guidelines. z12si20228264pas.77 - gsmtp

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This email address is certainly not being used for spam.

I think these two error messages are related but I'm not sure. Either way, I am unable to send emails from the Zoho email address through gmail.

Any ideas?