With 20/20 hindsight, if the FSF had used a P.O. Box number in the license, the license addresses would always be correct even if the FSF office changed addressed or (as now) was no longer maintained.

Of course, the cost of a P.O. box over 40 years would have added up to thousands of dollars and that is less money for FSF advocacy. And time spent going to the post office to check the box would also have taken away from advocacy time.

Another physical mail DNS-like idea is mail forwarding -- but it typically has time limits at the post office although not for private mail forwarders: https://en.wikipedia.org/wiki/Mail_forwarding "Private mail forwarding services are also offered by private forwarding companies, who often offer features like the ability to see your mail online via a virtual mailbox. Virtual mailboxes usually have options to get your mail scanned, discard junk mail and forward mail to your current address."

Although strictly speaking, these forwarding services are not quite like DNS (even if they do get at the idea of indirection). A true mail DNS would be more like a service you mail a post card to with a person's or organization's name and which mails a post card back to you which tells you what address to currently write to in order to reach that person or organization. (At least, if you write to that received address during some time-to-live window of validity of the address.) And I guess Encrypted DNS would be like you and the service using more expensive security envelopes instead of post cards? :-)

That's so strange, especially when you consider that for legal purposes, if you receive mail at someone's home, you are now a "resident" and it is harder for police to kick you out. Why would anyone willingly want your mail to come to your address.