I can’t use (external) private mail address for a user under the user settings.
UCS 4.4 or UCS 5.0-x
Login as root via terminal to your DC-Master (Primary Directory Node) and use the command below:
ucr set directory/manager/web/modules/users/user/properties/mailPrimaryAddress/syntax=emailAddress
After then a quick restart from the UMC
service univention-management-console-server restart
Now you will be able to use any kind (external) private mail address for your users.