I'm looking for a mail client that encrypts not only my mail ids and passwords but also downloaded messages. Possibly with mail-to-mail end encryption. It would be even better if the application was open-source. Is there anything like that or is it too much for my expectations?

I know that thunderbird encrypts my passwords and its enigmail plugin enables pgp usage but it doesn't encrypt downloaded data so even when the attacker doesn't know my gmails data, he/she can still access all the storage that was downloaded from the server previously.