Using procmail to forward inline - procmail

Question

I would like to use procmail to forward a message to another email address. Both the headers and body of the incoming message should be in the body of the outgoing message (inline forwarding).
Example incoming message:
From: outside#example.com
To: me#example.com
Subject: Test
Date: Mon, 03 Nov 2014 05:00:04 GMT
This is a test
The forwarded message should be like this:
From: me#example.com
To: thirdparty#example.com
Subject: Fwd: Test
Date: Mon, 03 Nov 2014 05:01:00 GMT
From: outside#example.com
To: me#example.com
Subject: Test
Date: Mon, 03 Nov 2014 05:00:04 GMT
This is a test
Can this be done using procmail, maybe in conjunction with something like formail?

Answer 1

Easy enough.
:0
* Some conditions, perhaps? Omit this line to forward unconditionally
* ^Subject:[ ]*\/.*
| (echo From: me#example.com; echo To: thirdparty#example.com; \
echo "Subject: Fwd: $MATCH"; echo; cat -) | $SENDMAIL -t
If you don't care about forwarding the original Subject header verbatim, this can be simplified additionally.
The -t flag to sendmail says to use whatever To: and Cc: headers are in the message to determine the recipient. I omitted generating a Date: because (most imitations of) Sendmail will do that for you.
The stuff in the square brackets should be one space and one tab, as usual.
If you want to keep a copy, either add Bcc: yourself (and take care to not have the incoming copy trigger a mail loop!) or change :0 to :0c which makes Procmail continue down the rest if the recipe file.