Q) Does this honour the "Receive news and updates from us by email" mailing list checkbox in user preferences?
A) as of v4.0.0a (the first public release), no - the email digest emails are considered separate to the newsletter emails. I might add a configuration setting which allows you to over-ride that behaviour and have "Receive news and updates from us by email" stop email digests as well when unchecked.
Q) Can a use click a link in the email to unsubscribe automatically?
A) yes, unsubscribe links are included and will set their digest preference to "None"
Q) Does the list of threads honour user permissions?
A) yes, the list of candidate threads is filtered for each user - they will only see threads they have permission to view
Q) How many threads are included?
A) Up to 10 "New" threads (ie created within the last week or last 24 hours, ordered by most replies), plus up to 10 "Updated" threads (ordered by most number of replies made in the last week or 24 hours).
Q) Which users receive the emails?
A) Users who are considered by XF to be "Valid", ie:
- not banned
- User state is "Valid"
- Users who are "Recently active" (which by default means have logged in at least once in the past 180 days)
- Users who have either explicitly selected "Daily" or "Weekly" in their digest preference
- Users who have NOT made an explicit selection, and the admin has set the Default Frequency to anything other than "None" (ie the admin is sending these emails out by default).