Thanks to you all,
If Grey will succeed it will be wonderful !!
As we are in progress of evaluating to switch from ISPConfig 2 to V3 it sure will be a thing I will look into, as we are happy users of your last RoundCube Package for v2 for a long time, Hans. Thanks for that again. So if I can help you this time, it would be a great "payback"
Thanks for your words and You'll have no regret when you're migrated to ISPConfig3!
The ispconfig mail_user table has a field server_id which holds the ID of the server this address belongs to. Yoou can lookup the server hostname then with tis ID in the server table.
I think that information is very useful indeed.