Don't send emails on behalf of FROM addresses we don't own

Created by: das-g

We are sending emails on behalf of the user (i.e., with their email address as FROM header). We send these emails to ourselves (thus urgency low), but still: If we want to continue to send email on behalf of addresses we don't own, we should at least also set the SENDER header to an address we do own. (And for which our server is seen as a legitimate email message source (tracked in #300 (closed)).) We should use a FROM address we own for sending email.