There may be times when a cPanel account sends more emails than the hourly limit assigned to them. By default, cPanel provides a buffer of 25% of the limit. The server will queue up to 125% percent of email messages (above the account’s hourly maximum) and then start discarding emails. For example, if the account's hourly limit is set to 100 and it tries to send 135 emails in an hour, the server will deliver 100, queue 25 for later, and discard 10.
The steps below cover how to change this percentage.
1) Log in to WHM as root and navigate to the following location.
Home » Server Configuration » Tweak Settings
2) Select the "Mail" tab.
3) Update the "The percentage of email messages (above the account’s hourly maximum) to queue and retry for delivery" option to the desired value.
4) Click the "Save" button at the bottom of the page.