These commands will suspend and unsuspend users from sending email from your server ...
// suspend outgoing email
whmapi1 suspend_outgoing_email user=$USER
// unsuspend outgoing email
whmapi1 unsuspend_outgoing_email user=$USER
replace $USER with the desired user
Notice : if you suspend and unsuspend the same user from WHM this restriction will be unsuspended also, so you must unsuspend outgoing emails again.
// suspend outgoing email
whmapi1 suspend_outgoing_email user=$USER
// unsuspend outgoing email
whmapi1 unsuspend_outgoing_email user=$USER
replace $USER with the desired user
Notice : if you suspend and unsuspend the same user from WHM this restriction will be unsuspended also, so you must unsuspend outgoing emails again.