Thank you for your reply. I have some question regarding your response:
What do you mean by the LDAP web interface? Do I need to install another LDAP gui? I didn't find any settings in the user management panel to set a mail attribute. Unless I misunderstood something (or looked at the wrong places).
I am aware that I can set this on the client (roundcube) via identities. However, there I have the problem that I need to set up every client from every user individually. Some of them use IMAP combined with Mail Software such as thunderbird or emclient. I need to set it on the server side otherwise there is no end to administration of all those devices (mobiles, pc's, tablets,...).