delivered_to header is not taken into account

Problem/Motivation

Some forward reply messages (like I'm out of office kind of mails) seems to use the delivered_to header, which is not yet taken into account.
Sadly there is no real standard behind that http://stackoverflow.com/questions/4742533/is-the-delivered-to-header-th....

Such a header looks like the following:

     [delivered_to] => Array
                (
                    [0] => stdClass Object
                        (
                            [mailbox] => John
                            [host] => doe.com
                        )

                )

Proposed resolution

Let's take that header into account as well.

Remaining tasks

User interface changes

API changes