I was moving some services to a different server and the postfix configuration wasn’t working right. The /var/log/mail.log had a bunch of repeated messages about
Jan 18 15:44:42 HOST postfix/trivial-rewrite[30797]: fatal: open database /etc/postfix/sender_maps.db: No such file or directory
Jan 18 15:44:43 HOST postfix/master[30461]: warning: process /usr/lib/postfix/trivial-rewrite pid 30797 exit status 1
Jan 18 15:44:43 HOST postfix/master[30461]: warning: /usr/lib/postfix/trivial-rewrite: bad command startup -- throttling
The /etc/postfix/sender_maps file existed so I just needed to enter this command:
postmap sender_maps
and then sender_maps.db was created and mail started going out!