You receive the following error when sending email:
Sender email@example.com has an outgoing mail hold.
The error is caused by the automatic blocking system which, when certain preset limits are exceeded, blocks the sending to prevent the IP or the sender from creating problems to their reputation, at this stage the outgoing emails are not deleted but are simply blocked in queue.
In this case, an analysis is required to determine the reason for the block and remove the cause before re-enabling the sending of emails.
The user lock is present in the outgoing_mail_hold_users