"IMAP has a perfectly good search feature"...yeah right.
The protocol has a reasonable search command, but have you actually tried using full-text search on any IMAP servers? I don't know of any for which it takes a reasonable amount of time to execute. Opening every single message in turn and looking for the text is not a useful implementation of fulltext search.
Furthermore, IMAP is a utterly useless at multiple-mailbox handling. Most every command, including search, operates on a single mailbox at a time. Of course, the user often wants to do a search across all mailboxes (and be notified of new mail in all mailboxes, but that's another discussion...)
Once the servers people use ACTUALLY have a perfectly good search feature, then, maybe, clients will start using it instead of their own local index.
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