Symptoms
Exim fails to send an email, and an entry similar to the following can be found in the Exim log.
T=remote_smtp defer (-53): retry time not reached for any host
Description
When too many outbound emails to a domain fail, Exim will defer delivery and attempt to send the emails again later. However, if the emails continue to fail, Exim will set a wait time before trying to send messages to that domain again. This is not necessarily an error. It just means that there were errors trying to send messages to a domain and that the wait time has not yet been reached before Exim attempts to send messages to that domain.
Workaround
The most efficient solution is to find out why the retry error occurs and resolve that issue. One easy way to determine the cause of the error is by executing the exinext command.
exinext recipient@domain.tld
Please note that "recipient@domain.tld" must be replaced with the recipient email address of a message that was deferred.
A mail server that is not accepting email will return a Connection refused error
mail.example.tld:$MAIL_SERVER_IP error 146: Connection refused
If this does not indicate the error, the next best option would be to read the Exim log 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 or clear the retry database.