[exim] Router decline locally generated messages

Hello,
How can I get a router (dnslookup specifically) to decline if the
message is locally generated.

I don't mind empty envelope from OR sending IP 127.0.0.1
Probably has to be the former.

I Tried
         senders = ! <>
But that does not work

I think
condition = ${if eq{$sender_address}{127.0.0.1} {no}{yes}}

is invalid because sender address is no longer defined in a router

TIA
Stuart