If Webmin is unable to send emails, for example, from automated backups, then following fix may work for you.
Before this ensure you have some kind of mail configured on your system. If not, here’s a guide to configure ssmtp
Basically, you need to go to Webmin Configuration and check “Sending Email” configuration. For me, it was a matter of setting “From” address. Quite simple, but took me hours to figure out..