exim

dpkg-reconfigure exim4-config
systemctl restart exim4

View logfile.

cat /var/log/exim4/mainlog

Force re-delivery of frozen emails.

exim -qff

View message details

exim -Mvh <message_id>   # View the headers
exim -Mvb <message_id>   # View the body
exim -Mvl <message_id>   # View the logs