If you cannot search for messages in Apple Mail (a search doesn't find expected results), try rebuilding the Spotlight search index for email messages.
Go to Apple menu, then "System Settings" On newer operating systems, click "Spotlight". On older (pre-Sonoma), click "Siri & Spotlight". On older systems, scroll down to search results on the right. Deselect "Mail & Messages." Wait for a few seconds. Restart your Mac. Repeat all the steps, but reselect the "Mail & Messages" option. The search for Mail should be rebuilt.
Note 1: The restart might not be necessary, but include it if you can. Note 2: After these steps it might take a while for the search to be fully rebuilt.
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