Return path incorrectly set

@lobo: this one is so weird, would like a code review.

Symptom: the "normal" emails are sent with the smtp envelope set to www-data@example.org

Cause:
In the pear Mail/xxx.php files, you got:

if (!empty($headers['Return-Path']))

{ $from = $headers['Return-Path']; }

But in CRM/Utils/Mail.php

$returnPath = CRM_Core_BAO_MailSettings::defaultReturnPath();
$from = CRM_Utils_Array::value( 'from', $params );
if ( ! $returnPath )

{ $returnPath = self::pluckEmailFromHeader($from); }

$params['returnPath'] = $returnPath;

So you set returnPath & get Return-Path. Clearly, didn't work.

I don't understand how to modify the defaultReturnPath (I want to empty it so the sender email address is used).

Anyway, today it's never used I think (a proper return path)

After the fix:

1) if you haven't configured your mailbox, it's going to use the from, that might work, or not (eg. if my email is @gmail, it won't)

2) the bounces are going to come back to the default mailbox (bounces), but as the verp isn't set, no idea what will be the result.

And

3) How to configure (somewhere) so it returns to the sender (because I know it's properly configured on my side)

Needs advice on how to proceed.