====== FIX: remote_smtp: all hosts for 'DOMAIN.TLD' have been failing for a long time (and retry time not reached) ======

This error in most cases means that retry database is corrupted. So remove retry files and restart Exim.

<code bash>
cd /var/spool/exim/db
rm -f retry retry.lockfile wait-remote_smtp wait-remote_smtp.lockfile
service exim restart
</code>