[exim] Replies to mailer-daemon@

Hello list,

I'm having a rather legitimate request from my users:

An order confirmation is being generated by a web app,  having the
sender webshop@???.

The customer might need some more info and hits reply to the email
above. Only the webshop's mailbox happens to be full, so the customer
eventually gets the following bounce from my mail server, with the
sender set as mailer-daemon@???:

A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:

webshop@???
mailbox is full: retry timeout exceeded

And here comes my problem:

The customer doesn't actually read the error message, he/she just hits
reply one more time, again requesting some data about his order.

But since the recipient is actually the postmaster, his second email
never has a chance and all it does is annoy the postmaster, who has no
involvement in managing the webshop, nor does he care about what people
order online. As a matter of fact he'd be more happy without this piece
of info...

There are obvious solutions to my problem:

I could increase the quotas, but as we all know this would be just
another arms race.

I might try to educate the webshop owners, but most of the times it
yields no results.

Is there a way I can make exim differentiate between "legit" bounces to
postmaster and these gratuitous replies ? Maybe look for empty return
path before accepting emails to postmaster ? Is this within the canons ?

How would you guys handle this situations ?

Thank you for your thoughts,
Marius