I guess there are ways to prevent such outcome by pre-selecting subscribers by level of technical knowledge on the subject.

An online test (chat like) is applied also by private torrent trackers to select uploaders also by tech knowledge of the subject before they are accepted. The same system can be extended.

Unfortunately, this is possible only where are many wanna-be subscribers to choose from the pool.

Hello

We are using for years home and business gnupg (for public/private keys) + pass + QtPass + git (for remote central storage). We are using it even as a team with shared keys.

Hello.

Have you tried an FOSS XMPP server solution ?

Just replace your entire Microsoft email server with a self-hosted FOSS Linux email solution, where you can have unlimited email accounts.