If one of your hosted user (or account) have accidentally reached the max deferred message that can be sent over an hour (and the issue have been identified and fixed), you can unsuspend the mail delivery by deleting the following file :
/var/cpanel/email_send_limits/max_deferfail_domain.tld
For example, if the domain name is “itechlounge.net“, the command to execute would be :
rm /var/cpanel/email_send_limits/max_deferfail_itechlounge.net
NOTE : The max deferred threshold feature is critical to fight against spam and avoid your IP address from being blacklisted. Finding the root cause prior unblocking is very important. It may be caused sometimes by an infected PC sending out emails without the customer awareness or a breach of policy on a specific target SMTP server. Browsing the mail logs and also looking at the mail queue would be useful in your analysis.