That works pretty well for IPv6, doesn‘t it (actually, it does not)? The problem with that approach is that as long as 0.01% still uses the old protocol, you c

That works pretty well for IPv6, doesn‘t it (actually, it does not)? The problem with that approach is that as long as 0.01% still uses the old protocol, you can‘t get rid of it. And then you have a bag with the old and new protocol and maybe after 20 years you realize it‘s going to take another X years until you have at least some relevant adoption, as demonstrated with IPv6. If one is ever going to „replace“ email it happens on top of the existing protocols and not as a replacement.