Q: Which server name should i use for the ionos groupware account?
A:
ionos uses a mixed system, the first email is a groupware account, the other are imap acounts (by default, upgrade possible). For the groupware account, you should use the servername exchange.ionos.eu, and for default imap accounts imap.1und1.de
For more tips regarding server names, please read the resoective FAQ article