This is a partial list of useful commands related to postfix/sendmail for my own reference. It's posted here in case anyone else finds this useful, too.
List stuff in the mail queue:
Read a message in the mail queue with example id 012345678F:
postcat -q 012345678F
Delete a message in the mail queue with example id 012345678F:
postsuper -d 012345678F
Delete ALL messages in the mail queue (warning - no confirmation nor undo!):
postsuper -d ALL