[Exim] Reject log: "cannot route to sender"??

Nearly all of the entries that were in my reject log have "cannot route to
sender" in them. I have:

receiver_try_verify = true
sender_try_verify = true
headers_sender_verify = true
sender_verify_reject = true
sender_verify_fixup = true

but the message is not about verification failure as such.  The addresses
look reasonable to me.  I cannot find the string  "cannot route to sender"
in the Spec or FAQ.  Where should I be looking to find out what is going
on here?
    Thank you,
    Hugh
    hgs@???