If you are receiving a message such as 'domain has exceeded the max defers and failures per hour' then this means that cPanel has detected that a certain percentage of your messages being sent are not being accepted by the recipients for one reason or another.
We have a percentage limit set for failed deliveries and should you exceed that amount in an hourly period, the server will then automatically block your outgoing mail for the duration of the hour and then re-enable outgoing mail again. Should the server see this activity continue, this restriction will be placed on the account again for another hour, and so this process goes on.
This restriction is in place to prevent the use of mailing lists that are not to correct/valid recipients. Should you purchase a mailing list for example and a number of the recipient addresses are invalid, this will cause your mailing to stop. You can see at any time any messages that are not being accepted from your hosting account using the 'Mail Trace' function in cPanel.