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If messages are in a Recovered Messages mailbox in Mail on Mac
If an email account in the Mail app on your Mac has a Recovered Messages Mailbox in the Mail sidebar, try these suggestions.
If Mail moved messages to the Recovered Messages mailbox when it updated your email library after a macOS upgrade, drag the messages into another mailbox.
If messages from a specific mailbox are repeatedly moved to the Recovered Messages mailbox, rebuild that specific mailbox.