Symptoms
If too many emails sent to a specific recipient are failing, Exim will wait before attempting to send to that domain again. Resulting in an error similar to the one below.
T=remote_smtp defer (-53): retry time not reached for any host
Description
If too many outbound emails to a certain domain are failing, Exim will "retry" to send the emails again at a later time. However, if the emails continue to fail then Exim will wait before "retrying" again. This is not necessarily an error, it just means that there were errors when first trying to send the message, and now the retry time has not been reached yet.
Workaround
The most efficient solution is to find out why the retry error is taking place and resolve that error itself. One easy way of determining the actual error is by executing the "exinext" command with the email address experiencing this error. Here is an example of the command below:
exinext testing-user@testing-domain.com
If this does not clearly indicate the error, the next best option would be to read your Exim log, located at "/var/log/exim_mainlog" or use WHM's Mail Queue Manager to determine why the first email to that recipient has been failing. After you have resolved why the email is not sending to that recipient, you can wait for Exim to retry the email at a later time.