Up until today, and new with Zentyal 4, the mail subsystem had to be restarted before email could be sent (TBird, webmail, Android).
Until today, Mail always displayed in the dashboard as "Running" whether it worked or not.
Today, it shows as "Stopped". The "Start" button displays that starting Mail was successful, but the status still displays "Stopped".
Yet I can send and receive email (for now).
Can someone explain why there are three mail subsystem categories in the web admin user interface? (OpenChange, Mail, and Mail Filter?) Did that really make sense to someone?
How do I troubleshoot "Mail" problems?
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SOLVED:
I had added an external email account (Google) to fetch email from.
Removing that account allows the Mail subsystem to display as "Running" and it still works.
This appears to be an identical bug to the one in the 3.5 release.
This still doesn't explain why there are three Mail subsystems in the user interface, but then I have not seen the UI team's automobiles: they might have separate Left-Turn, and Right-Turn steering wheels plus a Turn-Stick labeled with arrows pointing up and down. So, not enough data.