[exim] Controlling Backscatter

A quick summary of our setup.

We accept mail for customers, filter the mail and then forward it on to
their server.

To prevent backscatter we do not accept any mail for an email address that
does not exist at a domain.

A problem we run into however is if our customers servers (where we deliver
their mail) has spam protection (a 2nd level, which we do not recommend)
they may reject the mail. We often see stuff like this, when trying to
deliver to the customer server:

550 5.7.1 Message rejected as spam by Content Filtering.

We have no way to know if the destination server will accept the message
when we accept it on their behalf.

We trap those errors (delivery failures we call them) and report them to
the users. Our Exim server handles the undelivered and sends a bounce back
to the original sender.

If it is a problem like a full mailbox, temporary error etc. etc. we want
to send back an undelivered notice. In the case where our customer is
rejecting the messages because of some spam block we would rather not send
back any undelivered notice.

Essentially we want to control what undelivered notices we actually send.
Any way to do this? Selectively choose what undelivered notices to send
back to the original sender?