Thank you Dirk, that worked pretty well. Due to the Fact, that I'm Usinge Zarafa - the Homeserver Dropdown in the "Advanced" tab was allways empty, even if I used your suggested way via the Command Line. A short check showed, that the HomeMailServer was set right, but not shown in the Dropdown. So I decided to move the HomeMailServer Dropdown from the "Advanced" Tab to a Custom Attribute and set a new Tab "HomeMailServer". Here I can enter the Data directly and not via Dropdown and the Data stays in there.