Summary

/etc/msmtprc is setting "tiredofit/db-backup" as a domain. The forward slash (/) is an invalid character for a domain which can throw an an error:

smtp failed-command command="HELO tiredofit/db-backup" result="501 5.5.4 Invalid command arguments: Invalid domain name

Steps to reproduce

Enable email notifications, force a backup error

What is the expected correct behavior?

Email sent successfully via msmtprc

Relevant logs and/or screenshots

2025-09-27T08:10:36.261293-04:00 nas smtpd[176730]: ca1ad1f763f7ffbf smtp failed-command command="HELO tiredofit/db-backup" result="501 5.5.4 Invalid command arguments: Invalid domain name"

Environment

• Image version / tag: latest
• Host OS: docker image

Possible fixes

Modify /etc/msmtprc and change domain to a valid internal domain. You could also bind mount an alternate /etc/msmtprc from your local system. Ideally it should be changed upstream to 'local.domain' or enable users to change it via a environment variable.