Postfix setting reply-to field to sender

From the Postfix documentation, it appears you need to modify this section in your main.cf file:

http://www.postfix.org/ADDRESS_VERIFICATION_README.html

/etc/postfix/main.cf:
    smtpd_sender_restrictions = 
        permit_mynetworks
        ... 
        check_sender_access hash:/etc/postfix/sender_access
        reject_unknown_sender_domain
        warn_if_reject reject_unverified_sender 
        ...
    # Postfix 2.6 and later.
    # unverified_sender_reject_reason = Address verification failed

    # Default setting for Postfix 2.7 and later.
    # Note 1: Be sure to read the "Caching" section below!
    # Note 2: Avoid hash files here. Use btree instead.
    address_verify_map = btree:/var/lib/postfix/verify

This will put warning messages in your logs when a sender fails verification, but it won't reject it. You can remove the warn_if_reject line if you don't want to pollute your log files.

UPDATE:

If you want regular expressions, you'll need to compile PCRE support into Postfix (if it's not there already):

http://www.postfix.org/PCRE_README.html

Then, you can simply replace the line check_sender_access hash:/etc/postfix/sender_access with: check_sender_access pcre:/etc/postfix/sender_access and it will check for regex's in that file. If you're using Debian/Ubuntu, there's a package called postfix-pcre that will install support for PCRE.

http://www.postfix.org/pcre_table.5.html

A not so elegant solution would be to use REDIRECT in header checks. It will send your email to desired address but it will not rewrite the TO header.

In main.cf uncomment or put

header_checks = regexp:/etc/postfix/header_checks

if it is not already.

In /etc/postfix/header_checks put something like:

/^Received:.*app1.example.com.*/ REDIRECT testing@example.com

If reverse DNS does not work correctly you better use the IP instead.

Don't forget to run postmap /etc/postfix/header_checks