i finally made it and connected my user accounts directly to their IMAP accounts at 1und1.
So most of the users are happy now.
But for some reasons, some of the users do have e-mail addresses and IMAP accounts for more than one domain.
Since the recipients should get their mail with the 'right' sender, it is no possible setting up these different e-mail addresses as aliasses for one account at 1und1.

Is there any way to do this at OX?
I know, I could create more than one OX account for these users, but having the different e-mails accounts integrated in one OX account would be a better solution...