Did you by any chance add a new group where you experience this issue or remove a mail address?
Ensure that you can reply to tickets in the group via mail, where Zammad is not able to send notifications.
Also take a look into your production.log, it might hold hints on what’s wrong.
production.log show this: I, [2019-11-21T13:41:57.975347 #11702-46998715404520] INFO -- : Skip trigger (Auto-Reply (Neues Ticket)/9) because sender do not want to get auto responder for object (Ticket/1366/Article/5826)
What the hell? Where is that option set? I’ve even created new mail accounts to test this, and all of them have this option… That explains why no mail is sent, but I don’t get what this option is.
E-Mail is required for a group to send trigger based mails.
However, as you named it “nicht zugewiesen” I’m not sure if you’re mixing things up, because it smells like you want it to be a overview like a “folder” in your outlook.