How to write a recipe to get the real email sender - email

Question

I am trying to send email back to the sender using sendmail -oi -t, but somehow it extracted MTA, how to write the receipt to get the real sender since the email return-path and first From in the email header is MTA, not the real sender? So sendmail can deliver email back to: Sender: +xxxxxxxxxx#tmomail.net
or From: +xxxxxxxxxx#tmomail.net
Here is the procmail.log and email received:
...
procmail: Matched "+1xxxxxxxxxx#tmomail.net"
procmail: Match on "^Sender: \/.+"
procmail: Assigning "LASTFOLDER= (/usr/bin/formail -rA"Precedence: junk" \
-A"X-Loop:addr#mydomain.com" \
-I"To: $MATCH"; \
echo "$result"; \
) | /usr/sbin/sendmail -oi -t"
procmail: Assigning "PATH=/home/r/bin:/usr/local/bin:/usr/bin:/bin"
procmail: Unable to treat as directory "/var/mail/new"
procmail: Skipped "/var/mail"
procmail: Assigning "LASTFOLDER=/var/mail/r"
procmail: Executing " (/usr/bin/formail -rA"Precedence: junk" \
-A"X-Loop:addr2mydomain.com" \
-I"To: $MATCH"; \
echo "$result"; \
) | /usr/sbin/sendmail -oi -t"
procmail: Opening "/var/mail/r"
procmail: Acquiring kernel-lock
procmail: Notified comsat: "r#884017:/var/mail/r"
From btv1==909280b6006==+1xxxxxxxxxx/TYPE=PLMN#tmomail.net Mon Apr 11 17:09:24 2016
Folder: /var/mail/r
Here is the email header:
From btv1==9062f44d095==+xxxxxxxxxx/TYPE=PLMN#tmomail.net Fri Apr 8 00:19:01 2016
Return-Path: <btv1==9062f44d095==+xxxxxxxxxx/TYPE=PLMN#tmomail.net>
X-Original-To: recipient#mydomain.com
Delivered-To: recipient#mydomain.com
Received: from ch3p-tmo-mm3-sfw004.syniverse.com (chi-tmo-mm3.syniverse.com [x.x.x.x])
by mail.mydomain.com (Postfix) with ESMTP id 7D444222D0
for <recipient#mydomain.com>; Fri, 8 Apr 2016 00:19:01 +0000 (UTC)
Received: from tmobile.net ([10.166.3.161]) by ch3p-tmo-mm3- sfw004.syniverse.com with ESMTP id RuIW8dEKbs9H2Iyg for <r#mr4.biz>; Fri, 08 Apr 2016 00:18:58 +0000 (GMT)
To: recipient#mydomain.com
From: +xxxxxxxxxx#tmomail.net
Content-Type: multipart/related;Type="text/html";boundary="-boundaryRMS123"
Date: Fri, 8 Apr 2016 00:18:58 GMT
Message-ID: 20160308001858639184#mavenir.com
Sender: +xxxxxxxxxx#tmomail.net
User-Agent: iPhoneOS/9.2.1 (13D15)
X-Virus-Scanned: by bsmtpd at syniverse.com
Here is the last part of recipe:
:0hc
* !^X-Loop:old#domain.com
* ^Sender: \/.+
| (/usr/bin/formail -rA"Precedence: junk" \
-A"X-Loop:addr#mydomain.com" ; \
echo "$result"; \
) | /usr/sbin/sendmail -oi -t

Answer 1

You are instructing formail to generate a reply and it will faithfully attempt to do so. By default, the -r option tries to select the best one out of a priority list of headers1; if the message has a Reply-To: header (but no Resent-From:, and no Resent-Reply-To:, etc), it will generate a reply to that.
With -rt, you get RFC-compliant but somewhat less pragmatic behavior, but it doesn't really help here.
Anyway, if you know exactly which header you want to reply to, just use that.
:0hc
* ! ^X-Loop:old#domain\.com
* ^Sender: \/.+
| ( formail -rA"Precedence: junk" \
-A"X-Loop:r#mydomain.com" \
-I"To: $MATCH"; \
echo "$result"; \
) | /usr/sbin/sendmail -oi -t
This uses formail -r in order to get correct In-reply-to: and References: headers (as well as any previous X-Loop:) but overwrites the generated To: header with the one we captured from the Sender: header.
The \/ token in a Procmail regular expression causes anything after it to be captured into the variable MATCH. We use this to grab the value of the Sender: header from the incoming message.
http://www.iki.fi/era/procmail/formail.html -- this is from an older version, but this code hasn't changed in a loooooong time.