For https://geowebforum.ch we’ve developed
to send a weekly e-mail listing all posts since the last such e-mail to all members of the forum who didn’t opt out. This is separate (and, if users keep both enabled, additional to) the automatic “summary” e-mails sent to inactive users by Discourse by default.
The code could be adapted to send the mail daily instead of weekly.
What’s still missing is an opt-out link in the footer of the e-mail message. (Currently, users who want to opt out have to do so at /my/preferences/emails (i.e., for GEOWebforum at https://geowebforum.ch/my/preferences/emails), which isn’t as easy as we’d want it to be.)