[exim] verify recipe when forwarding for a non MX host

I have a mail server thats the front door and a semi hidden real mail
server for it to hand off the mail to.

So I want it to do

accept domains = +gateway_for_domains
endpass
message = unroutable address
verify = recipient

off course since localhost is main MX, that breaks and all the ways I
can find to modify the MX, isn't accepted in accept acl's.