Thanks Igor!
My problem description was quite vague so far. I set up email for a domain (occupy-occupy.de). Roundcube works, I can send email and receive within roundcube for the email address I created.
What does not work is my email client (it works fine with other domains — not set up under Plesk) — which might be a configuration problem. I am not sure where and how I can set up the mail server (like mail.occupy-occupy.de). The Plesk Domain mail settings don't give me an option. Is mail.<domain.de> a standard setting?
Cheers, Frank