Claws uses MH format which is nearly as standard as mbox (it's the same thing but with each mail in a different file, it's good for performance with large mailboxes). There are a lot of plugins for claws, and one of them enables to use mbox format and is available for Maemo (i didn't try it!)

Claws-mail is really a full featured & customizable mail client (as much as Thunderbird IMO), but it's really not suited to small screen and finger usage.

It's my main mail client, but it has required some training and it may not be for everybody - YMMV