This would be handy for web form submissions. Since the email has to be from the contact for the message to be attached to the contact record (no way to set the contact once the message is received), we make our form submissions set the from address as the client's email. Because it can't be verified that the email is truly from that client, it is marked as SPAM. If we could send the form submission from our email account, but set the reply-to field as the client's email address, then it would be nice if Front could match the contact by that reply-to field instead of the from field.