One Apple Mail feature nobody here has commented on was called Piano Mover internally. I don’t know the public marketing name. It lets attachments over 10MB in emails be replaced with iCloud hosted file references.

I wonder if "Piano Mover" is a reference to the Goon Show episode called "Napoleon’s Piano" [1]

If you like absurdly silly comedy; Harry Secombe, Peter Sellers and Spike Milligan, you should track these down. These were made in the 1950's for BBC Radio.

[1] https://goonshow.org/goonshow/s6p4/

the marketing name is "Mail Drop", and it is separate from iCloud mail; though it should only fire if there's a 20mb+ attachment on the email.

cloudd is used by a shit-ton of services though, so I'm not sure how to track it down without better tracing of what actually initiated the call, or the contents of the call.

edit: tomasf nailed it - https://news.ycombinator.com/item?id=49158388

I think every major email hosting provider (that's not a dumb IMAP only platform) has some variant of this.