[Exim] primary_hostname read from disk?

I was hoping that exim4 would let me do something like this:

MAINDOMAIN=${if exists {/etc/mail/mailname}{${readfile {/etc/mail/mailname}{}}}{$primary_hostname}}
primary_hostname = MAINDOMAIN

Is there any way not to hardcode the primary_hostname in exim.conf and read
it from some other file?

Marc
--
"A mouse is a device used to point at the xterm you want to type in" - A.S.R.
Microsoft is to operating systems & security ....
                                      .... what McDonalds is to gourmet cooking
Home page: http://marc.merlins.org/   |   Finger marc_f@??? for PGP key