It works now. I had to activate both options in the users “Mail forwarding settings”:
[X] Yes, deliver to mailbox
[X] Yes, forward to addresses
Now clean mails are forwarded to an external address AND spam mail is stored in the Junk folder only. This is, what I want BUT:
the clean mails are stored in the inbox too. I don’t want it stored in the inbox. How can I disable this without disabling the storing of spammails in Junk folder?
To be clear: I WANT spammails stored in the Junk folder and all other mails ONLY forwarded to an external address (without local storing).
OK it does not work. When I set it to forwarding mails, ALL mails, including those in the Junk folder are forwarded. I want only the Inbox mails being forwarded, not the Junk.