Here is how to solve the following error message: Domain has an outgoing mail hold. Message will be reattempted later.
grep the cPanel username / email address and if found remove it from the following files:
/etc/outgoing_mail_hold_users
/etc/outgoing_mail_suspended_users
/etc/email_send_limitsOr via WHM API:
whmapi1 unsuspend_outgoing_email user=USERNAME
whmapi1 release_outgoing_email user=USERNAME