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Staff
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If you have enabled the "Verify Recipient Address" option with an SMTP server, but recipient verification fails, possible causes include:

  • The SMTP server is not available.
  • The network connection is not reliable between the FortiMail unit and the SMTP server.
  • The server is a Microsoft Exchange server and SMTP recipient verification is not enabled and configured.
  • When the SMTP server is unavailable for recipient verification, the FortiMail unit returns the "451" SMTP reply code. The email would remain in the sending queue of the sending MTA for the next retry.


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