tag that can be present in any note

["read-receipt-requested"]

means a read receipt is requested from everyone.

["read-receipt-requested", "pubkey"]

this means a read receipt is requested from the specified pubkey. this can be repeated many times.

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and the read receipt...

kind: 2298 (for obvious reasons)

content: none

tags:

["e", original note id]

may be posted to the relays the original note was read from.