There is some undocumented limit for Subject to 26 characters.
Service responsible for sending emails is Messaged. You can see log in CLI/SSH with command
If there is some log record with sending email, there will be something like
Mon Feb 22 10:13:21 2021 user.info Messaged: Start from new event "Backup" "Switched to main WAN (Wired)"
Mon Feb 22 10:13:24 2021 user.info Messaged: Email sent successfully
If mails are not working, the second line is missing.
My problem was, that I used subject longer than 26 characters.
Use Subject in Events Reporting for mail max to 26 characters.