Re: The TO_ and "X-Rcpt-To:" field...

You could you substitute this regexp for ^TO_ where needed:

        (^TO_|^X-Rcpt-To:(.*\<)?)

That would effectively include matching of "X-Rcpt-To:" fields.

Or you could change the problem fieldname before it reaches the rest of
your filters, changing "X-Rcpt-To:" to "X-Envelope-To:", a similar
fieldname that is matched by the ^TO_ and ^TO tokens.

        :0fhw
        * ^X-Rcpt-To:
        | formail -R"X-Rcpt-To:" "X-Envelope-To:"

David Hunt