>>Many antispam filters will right away classify such an email as spam.
No, that’s not true, UNLESS the antispam filter resides as a local software in
the client's software as a plugin to the email client, which would of course
detect the "attached file" even its not technically an attached file.
Remember that it’s the end user client who renders the encapsulated email as an
attachment.
Technically it is not an attachment, and will not get detected as an attachment
in mail filters.
Its technically an email encapsulated in an outer email.
NDR also is technically the original email encapsulated in an outer email.
Technically, an encapsulated email looks like this:
From: exam...@example.org
To: exam...@example.com
Subject: Fwd: Hi!
Content-Type: message/rfc822
From: exam...@example.org
To: exam...@example.com
Subject: Hi!
Content-Type: text/plain
Content text.
Spam filters react on 'Content-Disposition: attachment; filename="test.txt"'
and similar things.
>>Conceptually, it's more like a HTTP proxy server.
No, because in the proxy case, it’s the "sender" who asks the proxy server to
"fetch" something from "receiver"s location. It’s the opposite way, the
"sender" trust the proxy, and know that the "content" is genuine if he trust
the proxy, thus the proxy bears no responsibility for the content from a
location he asked the proxy to fetch content from.
However, if an end user SENDS something via the proxy, let's say a forum post,
the proxy usually bears the responsibility for the content, which we have seen
in numerous court cases where a proxy have been used to hide the origin of
illegal content, where the proxy owner have knowlingly been a forwarder of such
a content (eg, been negligent and refusing to for example block forum posts
from his proxy).
------
You could see it more like a reverse proxy.
A reverse proxy is a proxy, which is configured to show a domain (let's say
example.org) and show another web page (let's say letsencrypt.org) inside. The
reverse proxy of course bears ALL responsibility for the content its "hosting"
via its proxy function, meaning if letsencrypt.org would accidentially host
something illegal, the proxy owner will take the blow for the content visible
via "his website".
Best regards, Sebastian Nielsen
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