Web/Android/iOS: Concerned someone might be able to intercept or snoop in on your email messages? ProtonMail is a free, open source email service that will protect your privacy and security. It's also dead simple to use. The Swiss-based service, developed by CERN and MIT scientists, came out of beta today, along with new Android and iOS apps.
ProtonMail stores all of your email messages encrypted, which means that even the company can't read your emails. Emails you send and receive with other ProtonMail users are automatically encrypted end-to-end, so even if someone intercepts the messages, they can't be read.
And you can also end-to-end encrypt messages with non-ProtonMail email addresses: When you create your message, click the encryption button and enter a password. The recipient of your encrypted email will get a link to the message and prompted to enter the password to read it.
Another handy feature is the ability to set an expiration date for emails you send, AKA self-destructing emails.
Although you'll have to use your [id]@ProtonMail.com email address to enjoy these security features, you can get daily notifications sent to your other email address if there are new messages in your ProtonMail account.
There are many other ways to encrypt your emails, including using PGP, but ProtonMail is probably the easiest method so far. The free version gives you 500MB of storage for one email address and up to 150 messages per day. Upgrade to the plus version ($US6/mo [$8/mo] or $US55/yr [$74/yr]) for more storage and email addresses, including the ability to use a custom domain.
You can read more about ProtonMail's security here or sign up and try it out for yourself at the link below.