[Exim] Conditional rewrite to null sender?

For any messages received by SMTP from a specific host, that have a
specific envelope sender, I'd like to change the envelope sender to be
<> (eg, the null sender), and have all of the conditions that implies
be set (eg, the message considered to be an error return message)

I've tried a conditional rewrite, but it doesn't seem to like rewriting
to the null sender..

If you must know why, this is to munge around an NT server until we can
eliminate it, which insists on sending its bounces with an envelope
sender of postmaster@<thedomain>.com.. The problem is that we have set
that NT server to smarthost through the exim/unix box, and we have a
special router that directs postmaster@<thedomain>.com to
postmaster@<ourdomain>.com, so when some spammers sends mail to a user
on the NT server that either doesn't exist or is over quota or
something, the NT server sends its bounce through the exim box, which
can't deliver it becuase the spammers address is fake or nonexistant,
so it then "returns" it to postmaster@<ourdomain>.com, which is
hideously "bad form"..

(Trust me, I am anxiously awaiting the moment I can crush this NT mail
server into oblivion - but I have to deal with it for now - its
IPswitch Imail, if anyone has any idea how to tell it to use a proper
null sender for bounces and eliminate the need for the above)


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