The "exceeded the max defers and failures per hour" message is a cPanel message that outgoing email is being held on the server until the number of emails that have failed or been deferred is below the email threshold.
To view, the threshold of 'max defers and failures per hour', you can view the values in the WHM interface:
WHM: Home »Server Configuration »Tweak Settings --> Search for 'defer'
The default value is 5 for all email accounts on the server.
This Tweak Setting is a server-wide setting. Increasing the value will affect all email accounts.
Below are a couple of reasons why the message may appear.
Possible reason: The domain has bad Email Deliverability, causing your emails to remain in the Email queue. Emails in the queue will retry to send themselves at regular intervals.
By default, Exim using the following rules when retrying email:
# This single retry rule applies to all domains and all errors. It specifies retries every 15 minutes for 2 hours, then increasing retry intervals, starting at 1 hour and increasing each time by a factor of 1.5, up to 16 hours, then retries every 8 hours until four days have passed since the first failed delivery.
Once you have enough emails in the queue, the resent email(s) will exceed the limit and cause the error to appear.
We would recommend ensuring the Email Deliverability is suitable for the domain using the following documentation:
Possible reason: The Contact Information for the domain is using an outside email address to receive notifications.
Account notifications emails are configured in the following location:
cPanel: Home -> Contact Information
Though usual to use an external email address, for example, @gmail.com, there are times when the account has enough notifications sent that the email recipient mail server may believe the emails are spam. This would be due to the volume and structure of the notifications.
We recommend using an email address that is already on the server to receive these notifications. It is the best way to avoid this particular situation.
The "exceeded the max defers and failures per hour" message has more possible reasons for it happening. You can find out more at the following documentation: