[EXIM] Sender verify problems

I'm having problems overriding sender_verify with sender_verify_except_hosts.
I've got a box that needs to accept mail from users who may not
necessarily be in the password file, but legitamately can use the box
name as the return address. I put the box's name in
sender_verify_except_hosts, but exim still insists on looking for the
user locally and rejects the mail. Is sender_verify_except_hosts broken
in this regard?

If I turn off sender_verify then the box is wide open to abuse from mail
with fake return addresses.

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