Easily end-to-end encrypt your e-mails with OpenPGP using Mailvelope or Enigmail for Gmail, Thunderbird, etc
E-mail by its ancient nature is open to be read by anyone managing a server on either end and often the transit layers are not even encrypted. This is why governments and corporations should not be sending anything classified or compromising via e-mail. Many corporate entities also route their mail (for spam and malware cleansing)...
