Why are there so many files in /var/spool/postfix/maildrop

It could be anyone's guess as to what was putting them there, but I would suggest reading up on maildrop here: http://www.postfix.org/QSHAPE_README.html#maildrop_queue

Also, what was the content of the message files? This should be a huge clue as to what was dropping them off to sendmail per the link above.

Most of the mails in /var/spool/postfix/maildrop directory are system generated. Hence even though postfix is disabled or stopped you may have thousands of mails in it. Most of the mails (99%) are generated due to crontab. When any job is executed and the jobs cannot run normally it will send an email to root. You can set MAILTO=”” variable at the start of your crontab file. This will also disable email alert. Edit/Open your cron jobs: $ crontab -e At the top of the file, enter: MAILTO=""