Using older versions of Outlook for Mac in business environments
Hello Christoph,
I'm Diane, an Office Apps & Services MVP specializing in Outlook and an Independent Advisor.
That is not available in the "new outlook" yet - it's basically a beta and they just added IMAP support to it. (Previously only Exchange accounts or Microsoft cloud accounts were supports.)
You can use Help > Contact support to report it or leave a suggestion in Uservoice.