mutt
although if you have 100k+ emails in a folder, particularly a non-archive folder I'd suggest you're doing it wrong
However, you must be willing to integrate it with other tools (a frontend, something like fetchmail, and something like procmail, etc.).
but yes, if you are stuffing +100k emails into one folder -- that is completely insane -- that's why you use procmail to sort your email into different folders as it arrives; I subscribe to a LOT of email lists and ctrl-D delivers a lot of magic when I don't want to or have the time to sort through a bunch of crap but