prevent leak of msg-id of original mail when forwarding a mail

When forwarding an email the mail header contains the msg-id of the original received email, this might be a problem to the forwarding person.

We should remove the following header fields in forwarded mails:

References:
In-Reply-To:
X-Forwarded-Message-Id:

There are two scenarios where this can happen:

• user hits the Forward button
• user hits the Reply button and replaces the recipients

btw: also the body contains parts of the original mail header (when hitting the Forward button) but this can easily be seen by the forwarding person and easily be modified/removed (we do not modify the body).