Can't change default "from address"

After further investigation, I found out that to reproduce this issue a postfix server must has smtpd_sender_restrictions with reject_sender_login_mismatch[1].

That configuration actually does the mail server to check if $message['headers']['Sender'] address is owned by smtp_username.

In drupal_mail is stated

    // To prevent e-mail from looking like spam, the addresses in the Sender and
    // Return-Path headers should have a domain authorized to use the originating
    // SMTP server.
    $headers['From'] = $headers['Sender'] = $headers['Return-Path'] = $default_from;

Drupal core suppose that site_mail address has a domain authorized. But, once phpmailer enable one to set an another SMTP server and SMTP username, the address must also be owned by SMTP username (if SMTP server configured in that way).

In this patch, there is another text area in the advanced section in configuration form called "From address". This information if filled will replace $message['headers']['Sender'] and $message['from'].
Note that making header-Sender and body-from the same avoid identity forgery. And some SMTP servers do it automatically using filters before sending the message.

[1] http://www.postfix.org/postconf.5.html#smtpd_sender_restrictions

Resending patch to testbot it after #1719434: PHPMailer doesn't pass in testbot.

I've been using it in production since my last post without problem, and ayalon has tested it. I guess it is pretty safe to set it as reviewed.

Besides it will be semantic wrong, the 'contact' core module create a site contact form which uses site mail as recipient. Probably, there are other modules that use site mail as it semantically suggests.