In cPanel, once a domain reaches the max fail/deff limit, it is temporary blocked from sending outgoing emails to other domains. cPanel stores block in files:
/var/cpanel/email_send_limits/max_deferfail_DOMAIN_NAME.tld
To unblock a domain, simply remove the file from the email_send_limits
directory:
rm /var/cpanel/email_send_limits/max_deferfail_DOMAIN_NAME.tld
or to remove block for all domains:
rm -f /var/cpanel/email_send_limits/max_deferfail_*