>> Yeah, I think I've seen MTAs that do that.
>
> They do. At submission, security gateway, and final delivery.
I see that Thunderbird will store attached files on Box or similar
services. although it just edits the link into the text of the message
rather than making it a separate MIME part.
You will also find cases where such links are themselves replaced with a
redirection through some sort of filter, in order to prevent the attack
where a message is sent with a reference to something innocuous that
gets by filter checks, but is subsequently replaced with malware.
IME these sorts of filter mechanisms work poorly, but good luck getting
rid of them.
Ned
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