Both Thunderbird and Outlook have plugins for PGP and S/MIME. You can use them to read/send mail from Gmail and Yahoo domains as well.

Edit:

If you're using them for an organization that uses Exchange/O365, the admin would need to make sure IMAP/POP protocols are enabled over TLS/STARTTLS, as opposed to using EAS. However, if you're dealing with mail in an organization, you're probably not using your own encryption keys to being with.