Choose the menu item Domain List.
Go into the Action menu for the respective domain by clicking the little gear icon and choose Forwarding.
Enter the the IP address or the hostname of your mailserver into the field Mailserver .
Conclude the modifications, by clicking on Update.
Click on Nameserver on the left side of the customer area and then on the modify button of the respective domain. In case there already exist MX-records for the respective domain, you can modify those.
To create new MX-records please follow the subsequent steps:
Click on Add DNS entry
Leave the Name field empty.
Choose MX for Type.
Please enter the hostname of the Nameserver into the field Value. You can, if necessary set a priority into the field Prio.
To Conclude the modifications, click on Save.
If your mailserver only has one IP address, enter an A record for the subdomain mail. Afterwards you can enter a MX- record for mail.xxxxxxx.de.