Originally Posted by till
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.
That's exactly what I was thinking when I wrote my answer. I wouldn't like to do a dig or NS-lookup via PHP, as that's something that can be abused "relative easily". Also a simple (even a nested) query should not decrease performance of the login that much, so I would rather rely on the ISPConfig DB than other data
I'm currently looking into setting up a VM for testing a few ideas.