Howard Miller

And... RTFM

(Mailcow is a wrapper around Postfix)

If your organisation was such that you required an MTA then I would suggest that you probably wouldn't be asking what the "best mail server for Moodle" is in this forum. You'd have been hired due to your mail administration skills.

I'm being slightly facetious, but....

I don't mean the recycle bin. Although this is relevant. Even when files finally do delete, they still hang around in the "trash" for a few days.

What's a "module group"?

There's no such entity as a module in Moodle.

"... but only as a session cache and not as a session handler"

I'm not sure I understand what you mean. If you set up Redis sessions then the sessions are stored in the database. As opposed to the moodledata area. What else are you expecting?