Thunderbird
Open Source
Spam filtration
Thunderbird, maintained by Mozilla (also the developers of Firefox), is a free and open source email, newsfeed, chat, and calendaring client, that’s easy to set up and customize.
Notes:
Thunderbird also helps to get rid of spam/junk emails by using a spam filtration system. For the average user, Thunderbird can be relatively hard to configure, setup, or use.
Additionally, Thunderbird features built-in privacy-enhancing features such as Do Not Track, remote content blocking, and more.
Mailvelope
Open Source
OpenPGP
Mailvelope is a browser addon that you can use in Firefox, Chrome, and more to encrypt your emails with OpenPGP using webmail providers. The browser extension provides end-to-end encryption for your existing email address.
Notes:
Mailvelope is completely open source, made in Germany. It is compatible with Gmail, Yahoo, Outlook, Mailbox.org, Posteo, Web.de, Freenet.de, GMX, and many more.
In regards to security, Mailvelope is evaluated by regular security audits, as they state on the homepage of their website.