When the Reply-To header is present for incoming emails we
want to use it instead of the from address. This is usually the
case when forwarding an email via a mailing list into Discourse.
For now we are only using the Reply-To header if the email has
been forwarded via Google Groups, which is why we are checking the
X-Original-From header too. In future we may want to use the Reply-To
header in more cases.
