Email is a non-starter for reasons such as not being in control over who the other party chooses as an email supplier (thus resulting in Microsoft being fed all email traffic).

So snail-mail is the winner. My snail-mail obviously gives a mailing address. From a practical standpoint, that’s all I need. But it would be good to show some kind of electronic means of communication in the letterhead. Not directly for practical use but more of an expression that says “I’m not a luddite but you need to fix your shit” (in so many words).

Requirements:

• must be secure. A low standard of security is fine; it just cannot be so shitty that giant surveillance capitalists can see and exploit the payloads.
• must not rely on any non-standard or proprietary protocols.
• must have at least one FOSS toolchain available.
• must be suitable for documents sent asynchronously.
• ideally a different unique address can be furnished to each recipient.

Candidates:

• XMPP
• onion e-mail (email service by surveillance capitalists cannot send to @*.onion addresses)
• (hypothetical) clearnet email address hosted by a server that blocks inbound MS & Google server connections
• fax number

One problem with the above candidates is I don’t think the 1st two options have any kind of aliasing (I only know of one onion email service that deliberately lacks a clearnet alias, and it does not have aliasing on the userid portion). So I would have to create many accounts and they would never actually get traffic. They would just be symbolic. And the third candidate does not even exist AFAIK.

Problems with the fax number: these are not cheap and I would need a fax number for different countries. Also fax services are gatewayed so some senders send an email to a fax service the dispatches a fax, in which case Microsoft would still see the payload.