Despite the development of more modern ways of communications, email has remained one of most commonly used solutions in official communications through the internet. Therefore, a large amount of important and sensitive information is still transmitted by email. On the other hand, the technology behind e-mail is not completely secure, it has a lot of security flaws, and the users have no control over who can access the metadata and content of their e-mail communication, especially when using email services such as Gmail, Outlook.com, Yahoo Mail, etc.
Emails can be encrypted using PGP (Pretty Good Privacy) , which is based on public key cryptography. You need to generate a key pair - a public key which you share with others and a private key which you keep secret - in order to exchange encrypted emails with correspondents.
If you use an email provider like Gmail or Outlook, you can encrypt your communication using Thunderbird, an open source email client with built-in OpenPGP capabilities, or by using the Mailvelope browser extension which works with popular webmail services. By using these tools you can easily generate a key pair for encrypting your email, or import existing encryption keys.
However, there are email providers, such as ProtonMail or Tutanota, which encrypt your messages automatically when they are sent between their users, and also provide ways to send encrypted emails to those using other providers.