Wietse Venema: The behavior that you seem to prefer (throttle down domains after 4XX reply to "MAIL FROM") is really a bug in the Postfix SMTP client implementation.
Postfix normally does not throttle down domains after 4XX reply to
"MAIL FROM". The bug is that Postfix treats server-side disconnect
after 421 reply as a "lost connection" error.
So you should not rely on this old bug forever. It will eventually
be fixed.
Wietse
