You might want to edit Modest so it doesnt startup automatically,

Here is how you do that, first make sure you have leafpad text editor installed.


Now look through the text file for all the things that begin with X-Maemo-Prestarted and delete them entries.

This will stop modest from automatically starting itself. It will only open in the background when you connect to the internet or it you open it manually.

Give that a try, also let me know how using htop went to find out the application draining ur memory