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Searching email in local mailbox is a constant problem for me.

Nothing really support IMAP search.

Every mua sucks at large mbox file.

-- Edit: I am a happy fastmail user

FWIW I use Apple's default Mail apps w/ FastMail IMAP and don't have issues with search.

But I guess you mean search w/o local caching

Thunderbird's search works just fine for me with tens of thousands of mails.