I work for an ISP, and we provide a linux exim mail server as a backup mx for our clients. The problem is that we are now being listed on backscatter lists as we are sending User unknown replies when we get that reply from the customers email server.
This works fine most of the time but when forged 'from' email is sent our exim server is replying to that from address even though that from address didn't send the email in the first place.
How exactly can we fix this? We still want to provide a backup mx to our customers. Is there a way that we can configure exim to check a user exists first by opening a rcp session to their mail server before sending on email?
We cannot disable NDR's as that will prevent legitimate senders from getting NDR's to an old address.
Has anyone got any experience of this scenario at all?