Ubikom is free, open-source email framework based on the concept of Self-Sovereign Identity. You own your identity (which, in this case, means your private key)

Ubikom is free, open-source email framework based on the concept of Self-Sovereign Identity. You own your identity (which, in this case, means your private key), and all the outgoing and incoming messages are encrypted and signed by default. It works with any email clients that support POP3 and SMTP.