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
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.
The most efficient solution 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.