Problem
Recipient verification through SMTP fails.
Solution
If you have enabled the Recipient Address Verification option with a protected domain’s 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 SMTP server does not support ESMTP. EHLO, as defined in ESMTP, is a part of the SMTP verification process. If the SMTP server does not support ESMTP, recipient verification will fail.
• 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.