Replying to @pwaring
Personally I ensure /etc/mailname is set right and/or remember to set MAILTO in my crontabs

Aug 24, 2021 ยท 7:22 AM UTC

1
Replying to @dsilverstone @pwaring
If you're using a default Debian config though then /etc/hosts needs to be set up properly so that uname() followed by gethostbyname() returns the correct FQDN