Exim defer (-53): retry time not reached for any host

From time to time you may notice that some addresses give a defer error: retry time not reached for any host.
1Ruz3Y-0005TQ-Ek == email@domain.com R=dnslookup T=remote_smtp defer (-53): retry time not reached for any host
So, you need to find out what smtp server is used to manage address.com’s email. To do this — use the host command or more advanced dig, whatever you like best. This command should be issued on your mail server:

host domain.com
domain.com has address X.X.X.X
domain.com mail is handled by 50 remote.smtp.server.

First of all make sure that remote smtp server is accepting connections on 25/tcp and is operating without any unpredictable errors.

telnet remote.smtp.server 25
Trying x.x.x.x...
Connected to remote.smtp.server.
Escape character is '^]'.
220 remote.smtp.server bla-bla-bla

Next step is to tidy up exim retry database:

/usr/sbin/exim_tidydb -t 1d /var/spool/exim retry > /dev/null

In some cases you may want to delete all the retry entries:

rm -rf /var/spool/exim/db/retry
rm -rf /var/spool/exim/db/retry.lockfile