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@evan this open issue raises similar questions and @trwnh advocated for sending a separate activity for the bto/bcc'd user

https://github.com/w3c/activitypub/issues/326

For simplicity's sake, disallowing bto/bcc in S2S seems wise.

If blind recipients are omitted then it presents a real problem in S2S communications, and sending the activity to those users' inboxes directly ASSUMES behaviour on the other server's part, which is unwise.