Just figured out why msmtp wasn't working under Debian. Its AppArmor profile is too restrictive for how I'm using it. Leaving a note here for my future self and anyone else with the same problem. Credit to https://stackoverflow.com/questions/62838673/awk-permission-denied-when-run-through-msmtp
The solution:
sudo ln -s /etc/apparmor.d/usr.bin.msmtp /etc/apparmor.d/disable/
sudo apparmor_parser -R /etc/apparmor.d/usr.bin.msmtp
2 years ago
https://stackoverflow.com/questions/62838673/awk-permission-denied-when-run-through-msmtp