[Exim] rewritting Sender (exim4)

Hi,

For all mails locally generated by http user I want to rewrite
Sender to value from From/Reply-to so for example:
Sender: http@???
From: xyz@???
into
Sender: xyz@???
From: xyz@???

begin rewrite

http@???      "{$reply_address}"              E

Unfortunately $reply_address is empty. What did I miss?
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