That's only part of it -- use the system contacts database, integrated with the system mail settings, etc. I tried Claws, and wasn't too impressed with the experience as a whole. Similarly I'm not interested in whole gui replacements like gnome or kde. I like Hildon, but they've either got to improve the bugginess of the built-in stuff, or get out of the way and either let the community fix things like Modest, or develop drop-in replacements that can be integrated.

I used to use a Palm Tungsten C. The native Palm email client was lacking, so I went with SnapperMail. It was very integrated, essentially with the palm providing fundamental services that made SnapperMail a very effective drop-in replacement.