Open the item in that Sent folder and see if the items appear or REPLY to ALL on that sent item and see if it appears.
Bob
For example, I sent a message addressed to my two daughters. That is, both their addresses were in the To field when composing. But in the Sent mailbox the address of only one daughter appeared, so I have no record that I sent it to the other. I know that I did because that (missing) daughter "replied"!

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