RFC 5321 - Flag to disable Return-Path header

Per RFC 5321 the return path header should only be included in the message when sent by final delivery client/server, which is very unlikely to be PHPMailer.  Thus a flag was added that will disable the Return-Path header, while leaving the default to send it as always, thus maintaining backwards compatibility.

Some modern SMTP servers may reject messages containing the Return-Path header, such as Haraka.



Resources:
https://tools.ietf.org/html/rfc5321
http://postmastery.net/2013/05/13/about-the-return-path-header/
http://haraka.github.io/