End-to-end encryption (E2EE) is a way of encrypting email contents so that nobody but the recipient(s) can read the email message.
The standard way to do email E2EE and have it work between different email providers is with OpenPGP. There are different implementations of the OpenPGP standard, the most common being GnuPG and OpenPGP.js.
-There is another standard that was popular with business called S/MIME, however it requires a certificate issued from a Certificate Authority (not all of them issue S/MIME certificates). It has support in G Suite Enterprise/Education and Office 365 Business or Exchange Server 2016, 2019.
+There is another standard that was popular with business called S/MIME, however it requires a certificate issued from a Certificate Authority (not all of them issue S/MIME certificates). It has support in Google Workplace and Outlook for Web or Exchange Server 2016, 2019.
Email providers which allow you to use standard access protocols like IMAP and SMTP can be used with any of the email clients we recommend. This can be less secure as you are now relying on email providers to ensure that their encryption implementation works and has not been compromised in anyway.