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FAQ
hostname --fqdn did not return a fully qualified name
fix /etc/hosts etc, but hostname -f/--fqdn, check systctl -a|grep hostname if fqdn is missing there, run hostname mail.example.com and save yourself a lot of headache
Helo command rejected: need fully-qualified hostname
Make sure your FQDN/IP are ok in /etc/hosts
Message is frozen
Mail can't be delivered locally, check storage.
all relevant MX records point to non-existent hosts
This may mean the MX IP is a private one, in that case remove that IP/network from ignore_target_hosts
