[Exim] rewriting rule for non-existent Reply-To: field

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Exim users:

I have a need to add a Reply-To: field on incoming mails that are being re-written. The problem that I'm running into is that the incoming mails do not have a Reply-To: field at all.

How can I do this?

Thanks in advance for your help,
Tim Lank
timlank@???
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