have you use gmail offline lab facility ever ? Just see it how it is managing offline data... You can see all data with attachment will be on your local machine... It might be helpful for you... It is working same like you are online... No difference between online and offline.. User will have no problem if you will do same like gmail....
I am working on that currently. but my purpose is different... I am just sync offline and online data... You can store your email data in offline database and it can serve offline....
