This article describes how to troubleshoot if the LDAP user query has failed.
Scope
FortiMail.
Solution
The Fortimail LDAP user query has failed for some specific users.
Do a packet capture in the FortiMail, and do the LDAP query again. If the LDAP server does not respond correctly with the user CN, proceed to the next step.
Check whether the name of the user CN is more than 20 characters or not.
In this case, the user CN has 21 characters which caused the issue to happen.
Try to make the user CN to be shorter than 20 characters and verify the result.
Check the AD user, and make sure the email field in the AD user object is already filled with the email account (attached screenshot as an example) then run the LADP query test again and it should work
