WHM brings up this message:
This message was created automatically by mail delivery software. A message that you sent could not be delivered to one or more of its recipients. This is a permanent error. The following address(es) failed: user@example1.com (ultimately generated from website@example2.com) Domain example2.com has exceeded the max defers and failures per hour (1000/1000 (92%)) allowed. Message discarded.
There is a special place on the disk where you can lift this hold. Check here is the user’s email has been added, and if so, remove it:
/etc/outgoing_mail_hold_users