[Exim] primary_hostname, qualify_domain and mailer-daemon

I have a host that is both a webserver and a SMTP server. I would like
to set qualify_domain to the name of the webserver for mail sent
locally, yet I would like to set the domain used in mail bounces
(mailer-daemon@xyz) to be the hostname that people use to connect to
this server for SMTP service.

So it struck me that the string used to build the hostname portion
of mailbounces (mailer-daemon@%s) should not be qualify_domain, but
primary_hostname instead. Does that make sense, or is it a bad idea?
Perhaps a new config option 'errors_from_domain' would be in order ?

Mike.