I once spent a little effort - but not enough - trying to get offlineimap running on my N800 - it works well with GMail, is effectively just a set of python scripts, and stores mail in a format that Modest can understand. It does two-way sync, so changes you do locally will get sent back to the server. I use it on a Ubuntu box to keep a local copy of my GMail messages.

I seem to recall there was a dependancy issue, and my Maemo guts skills are not really up to par to begin with, but it may be worth looking at.

The usage would be that Modest just interacts with a local sorage, and offlineimap synchronizes with the server on a demand and availability (of WiFi link) basis.

Joe.