[Exim] Weird behavious WRT Sender: header

I have exim 3.33 running on a freebsd box. There is a script being
called, which uses BSMTP to submit a message to exim.

I have added no_local_from_check, and the script itself inserts a valid
Sender: header to the message. (This is coming from a www form, and the
From: may or may not be valid)

In one specific case where the From: is invalid, and this server goes to
send this message to another Exim machine (also 3.33, on linux), the
remote server rejects the message 'no valid sender in any header' (i
have header verifications on and want to keep it that way). BUT, it
looks from the logs on the receiving server that the sending server is
stripping out the Sender: header I have inserted.

If I make a telnet session to port 25 on that server, and insert a
message with the same invalid From: header, but include the same valid
Sender: header, it accepts the message fine.

exim -Mvh on the message on the sending server DOES show the Sender
header in place and intact, it just doesnt seem to get sent with the
rest of the message. I modified the script to add an X-Sender: header
also, and it DOES go across.

Any ideas?

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