Some sendmail commands I’ve just found useful, but couldn’t locate on Google.
Get the current status of a message in the queue:
grep 'email@example.com' /var/log/maillog
Get the headers for an email:
cat /var/spool/mqueue/qf[message ID]
Get the body of an email:
cat /var/spool/mqueue/df[message ID]
(Many thanks to my old friend Mr Matt Westby for his help with this.)