Removed exim errors_to to support custom Return-Path

* Required to implement VERP
* Now, Return-Path is fetched from the 5th param to the mail()
function
* Helps in identifying failing recipients.
Removed the mail.ini file, as its settings become obsolete after
the commit

Needed By: I9463ae33ae327405725ea9693d45ad61b8ecf798
Change-Id: Ibb88a6cfd762f834340f6362fe70ea7e6c838b37
Merge "Removed exim errors_to to support custom Return-Path" into production