I have a user his email gets bounds back.
The recipient's e-mail address was not found in the recipient's e-mail system. Microsoft Exchange will not try to redeliver this message for you. Please check the e-mail address and try resending this message, or provide the following diagnostic text to your system administrator.
The following organization rejected your message: mail.shareplate.biz.
Diagnostic information for administrators:
Generating server: polonium.mailguard.com.au
mail.shareplate.biz #<mail.shareplate.biz #5.1.1 smtp; 550 5.1.1 User unknown> #SMTP#
Could someone please explain how to troubleshoot this issue?