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hostname --fqdn did not return a fully qualified name

fix /etc/hosts etc, but if hostname -f/--fqdn fails, 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