Problem
SMTP clients receive the message 451 Try again later.
Solution
There are several situations in which the FortiMail unit could return the 451 Try again later SMTP reply code to an SMTP client. Below are some common causes.
• The greylist routine has encountered an unknown sender or the greylist entry has expired for the existing sender and recipient pair. This is an expected behavior, and, for legitimate email, will resolve itself when the SMTP client retries its delivery later during the greylist window.
• Recipient verification is enabled and the FortiMail unit is unable to connect to the recipient verification server. There should be some related entries in the antispam log, such as Verify <user@example.com> Failed, return TEMPFAIL. If this occurs, verify that the server is correctly configured to support recipient verification, and that connectivity with the recipient verification server has not been interrupted.