On 09Feb18, John R. Levine allegedly wrote:
RFC 822 may not have versions but 821/2821/5321 sure do.
As soon as 2821 added EHLO, SMTP got service extensions and pretty
much by their nature, those extensions are not backward compatible.
Well-known examples are 8BITMIME and SMTPUTF8. If you have a message that
one of those and the server you're trying to send it to doesn't support the
extension, you lose, the message bounces.
I'm not sure whether this an argument for or against versioning. Or an argument
that sometimes bad engineering choices are made regardless of the upgrade
In any event, 822 is an existence-proof that decades-long upgrades are entirely
possible without the scorched-earth approach of versioning. I hope it would take
a very strong argument to suggest that we shouldn't (or can't) follow the 822
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