[exim] router precondition inversion

I want a router to not succeed if a envelope sender is a specific
match. More specifically, the router logs messages to BSMTP and uses
the 'unseen' directive to continue processing. I want to ignore
messages from certain senders. I had hoped to use

senders = !somedomain.com

but it doesn't seem to be having the intended effect(actually it
doesn't seem to have any effect).

should I be using 'condition' instead?