You can fix this through this command.

Code:
whmapi1 unsuspend_outgoing_email user=$USER

Ex : whmapi1 unsuspend_outgoing_email user=test

Here test is your cpanel account name

Ref :

https://forums.cpanel.net/threads/domain-has-outgoing-email-disabled.548681/
https://forums.cpanel.net/threads/suspend-sending-outgoing-email.651981/