Mail.app – About reindexing messages

If there are problems with information in your mailboxes, it may be necessary to reindex your messages. During this process, certain Mail folders on your computer are moved to another location, such as your desktop. When Mail is reopened, it rereads and reindexes all messages in all local mailboxes. The reindexing process can take several minutes depending on how many messages are in your local mailboxes.

When Mail detects a problem with your mailboxes, it may alert you that it needs to repair them by reindexing your messages. At other times, you can reindex messages yourself. To do so, quit Mail and, in the Mail folder in your home Library folder, move the Envelope Index file and any folders that begin with “Mac-”, “Exchange-”, or “IMAP-” to a different location, such as your desktop. When you reopen Mail, it reindexes all messages.