Configure mail servers to protect users' privacy

[gderber]

Checklist:

• [ ] Remove unnecessary headers #463
• [ ] Encrypt emails at rest #291

Refs:

• https://tools.ietf.org/html/draft-marques-pep-email-02
• https://wiki.archlinux.org/title/Postfix#Hide_the_sender's_IP_and_user_agent_in_the_Received_header
• https://linux-audit.com/postfix-hardening-guide-for-security-and-privacy/
• https://askubuntu.com/questions/78163/when-sending-email-with-postfix-how-can-i-hide-the-sender-s-ip-and-username-in

[gderber]

The pEp standard focuses mostly on the mail client, not the server.