Need a mail app for LARGE volumes of mail...

[Jim March]

I've been using GMail through the web interface.  I've just realized
it's dropping data.  It lost something important and I can't have
that.  I'm going to have to suck it all down locally.

I need a mail client that will handle massive piles.  And it has to
allow streaming it all in continuously.  We're talking about almost
6gig.  Thunderbird emphatically won't cut it.  Dunno about Evolution
but I suspect not.  I'm on Maverick.  I have enough embedded
graphics/PDFs/etc. to deal with that I need something GUI vs. terminal
interface.

Any clue what I should try?

Jim