[exim] using @mx_primary in conditions

Hello,

I'm using the @mx_primary virtual domainlist in a router. But the
domain-condition is not detailed enough. I have to specify a more
precised "condition = ..." instead of "domains = @mx_primary". How can I
use the @mx_primary domainlist in such a condition? Usually I'm using
lsearch-statements in textfiles.

Regards
Marten