Something like ... mozilla hunderwhatzit for email - backend that I own and front end that is at least somewhat user friendly
Discussion
You have to differentiate between the mail server, which should support IMAP and SMTP, and the email client, which should support PGP encryption.
The mail client can be some desktop software like Mozilla Thunderbird or a web based client where you use your browser to login.
For Linux, there's a lot of email servers. Like these: https://cloudzy.com/best-linux-mail-servers/
And Web based clients could be these for example: https://medevel.com/15-selfhosted-os-webmail-clients/
Anyhow, you will need some serious knowledge and work to configure everything.
Also, most commercial mail providers don't accept emails from unknown sources, so you will have to use some trusted mail provider to forward your own mails.