[extraduder_ire]

Is there an RFC for email to redirect email for a user no longer at that address? Not exactly like setting up mail redirection with the postal service, but similar in outcome.

e.g. a server connects to the gmail MX server, and gets a response like "[e-mail] now found at [e-mail]"

There's probably a ton of issues with this approach, but it would make switching email providers simpler on the user-end.

[vel0city]

Most email platforms support some form of forwarding. Its not quite the same as your suggestion that's similar to an HTTP redirect but still the ability to configure your email user to just pass along those emails to another address is a common feature. These systems usually just rewrite the envelope recipient address and reprocesses the email based on that new address.

In the end though this still requires that original user to have exclusive ownership to that username in perpetuity and requires the email hoster to continue to actually host email services. It does nothing if, say, Google wanted to shut down email services on @gmail.com or start requiring paid accounts or whatever.