>> 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.
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