If you receive an error message like this “Domain my_domain.gr has exceeded the max defers and failures per hour (5/5 (100%)) allowed”, means your outgoing mail has enabled a server rule, that will not allow you to send more emails. This occurs when a domain sends emails that are either rejected maybe due to an invalid e-mail address, or are being placed at a pending state from the recipient. The server allows a certain limit of such messages per hour. If this percentage is exceeded, then the server shall not send other emails for the rest of the remaining time (1 hour).
For example, 5/5 and 35% failure means that 5 messages have been unsuccessfully sent and at the same time the failure rate in relation to all the outgoing messages is 35%. Another case would be, if you sent 100 messages and 5 of them are failed, then the failure rate is 5%.
cPanel blocks the user from sending messages when the failure rate exceeds 20% and there are also 5 failed messages during the last 60 minutes. In case, for instance, your domain was blocked at 14:00, you will be able to send an email again at 15:00.
