Rewriting just a subset of mail header addresses

Hello all,

Does anyone know of a way how to address-rewrite just a subset of mail
headers with Postfix? Per default, all headers (From:, Reply-To:, To:,
etc.) are rewritten.

For a custom email-setup I need to rewrite the address in the From:
header line, while leaving the address in the Reply-To: header line
(if any) alone. Preferably, in case a Reply-To: line is missing, it
should be added with the original address as was present at the From:.
For the curious: at the end is described what all this is good for.

I'm afraid I won't come by a custom after-queue content filter to get
the above done, but maybe you have some suggestions for (part of) this
functionality (apart from hacking the Postfix code of course :-).

Thanks!

Jan Stap

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The why

The relay above serves as a front-end relay to a Lotus Notes gateway,
dedicated to the SMTP-to-Notes direction. Let's say at this network,
the email domain customer.com is in use. Some portion of mail coming
in from the Internet carries user@customer.com as the message sender
(e.g. from mobile workers of this company).

Without any rewriting, Notes will convert such senders into the
corresponding Notes email address and the recipient cannot tell
whether the message was internally generated, or came from outside
(this is what this customer wants to be able to distinguish). By
rewriting the sender to user+something@customer.com, Notes is fooled
and displays this exact address to the recipient. Now he can tell the
difference, but he also looses the ability to reply to such mail.
Having a Reply-To: address line with the unmodified sender address
fixes that.