To reindex messages:
1. Quit Mail, if its open.
2. In the Mail folder in your home Library folder, rename the Envelope Index file. For example, Backup Envelope Index.
3. Move the folders that begin with Mac-", Exchange-", or IMAP- to a different location on your computer, such as your desktop.
4. Open Mail.
Mail creates a new Envelope Index file and retrieves the account folders again from the mail servers. This process may take a few minutes, depending on how many messages Mail is reindexing.
5. Delete the renamed Envelope Index file and the account folders you moved to your desktop.