Handling thousands of mails on more than a dozen accounts with the only bayesian filter that beats Apple's own (and started earlier than them*), Powermail has been my baseline for years.
Once that I was setting a linux emailer unproperly and its first connection erased more than 1000 mails from a server where I did want to leave them, I connected through IMAP with Powermail and re-uploaded everything, in a single pass.
I just never found a software as powerful than Powermail.
(*) Powermail search capacity also is instant among Gbytes of mails (and compatible with Spotlight, of course)