Mail is text. Industrial strength mail handling is best done in text mode.

Some of my largest folders have over 20,000 items in them, and PC-Pine has no problems. PC-Pine conveniently allows mouse use on the text interface, making it essentially painless.

There is a version of Pine for OSX, but I don't know anything about the feature set. I'd still recommend it - I've been using Pine for nearly 10 years now on various platforms. It just doesn't give me grief. It also provides far more flexibility than any other mailer I've seen (does your mailer have Reply Role Rules?).

On occasion it has been known to do a strange thing or two - but because the underlying mails can be viewed and copied with a standard text editor (large files!) even the total loss of Pine wouldn't cut you off from your mail.