Mail being held
The cPanel servers are configured to hold mail where it thinks there might be a spam outbreak.
If mail needs to be released, you can see the Mail Queue in the left hand menu of WHM to release or delete the appropriate mails.
To allow a user to send mail again, try the first option below (allowing the user to send) and if that doesn't work or it's already allowed, use the API call
- https://documentation.cpanel.net/display/74Docs/74+Release+Notes#id-74ReleaseNotes-Rejectandholdoutgoingmailforsuspectspammers
- whmapi1 release_outgoing_email user=<username>