Hi,
Just thought I'd add to this list: Jumble (
Best secure email service, simple email security | Jumble is a free, integrated, end-to-end email encryption solution that integrates with existing email systems. Right now Gmail users can sign up and use Jumble for free for life (business users pay a small monthly subscription). Importantly and unlike some other providers listed here, Jumble doesn't ever hold a copy of your email data and we don't have access to your encryption keys.
We also support iOS and Outlook.
Want to see just how easy it is to get started? Watch this short tutorial video:
Note: regardless of whether your a personal or business user, decryption of messages will always be FREE.
Full disclosure: I'm one of the founders of Jumble