What it does
Stalwart is a self-hosted mail and collaboration server with user-controlled message-storage encryption. Its documentation explains encrypting incoming messages with a recipient’s OpenPGP key or S/MIME certificate.
Stalwart’s relevant feature is more specific than ordinary TLS. The server can encrypt delivered plaintext messages with each recipient’s registered public key before writing them to storage. A private key is then needed to read the protected contents.
The documentation separates delivered mail from messages appended by a client over IMAP or JMAP. Appended messages remain unchanged by default unless the relevant setting is enabled. An administrator should decide on both behaviors and the users’ key-management workflow; encrypted storage alone does not make the original sender’s ordinary SMTP message end-to-end encrypted.
Strengths
- Per-user OpenPGP keys or S/MIME certificates can protect stored message contents.
- Delivered and client-appended messages have separate documented controls.
- A community open-source edition is available.
Watch out for
- Client-appended messages are not encrypted by default under the documented setting.
Best for
- Mail administrators who can operate a server and manage user encryption keys