Reviewing email delivery configurations I found out that we were defaulting to daily digests (instead of the default weekly digests) all along. I have changed to the default. and new users will get a digest when not visiting the site for a week.
Now, what do we do with all the current users? Should we set weekly digests for everyone, assuming the risk that someone wanted to have daily summaries indeed? They will have to change the periodicity in their user profiles.
I bet a daily digest (that is, a digest every day you don’t visit the site) is excessive for most. At the same time, I bet many users will mark these digests as spam before checking that they can change their preferences in Discourse. Coincidence or not, today I have sent a test email from Discourse to myself, and it has ended in my spam folder (the regular notifications land in the correct inbox/folders).