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[Moin]

Did you check which frequency they choose in their preferences?
You can also check these conditions:

@Discourse

Why isn’t Discourse sending an activity summary to a user who has the setting enabled?

If the user has been active on the site, a summary has been sent recently, or their email has been bouncing, then they won’t be sent any activity summary. Inactive, suspended, or staged users will also not receive activity summaries. (If you’re still curious, you can see the full logic in the source code here ).

The only other reason they wouldn’t receive the digest is if the user was created via the sync_sso route and has never visited the forum. See Summary emails, new users and SSO - #6 by simon for details about that.