If the MAIL FROM doesn't match the From header,
we do not reject the mail, as this can be caused
by e.g. SRS forwarding. Instead, we reset the envelope
address, so it is reinjected as `MAIL FROM:<>`
to prevent sending a bounce message.
Closes#67
Signed-off-by: Jagoda Ślązak <jslazak@jslazak.com>