punished?

If you are talking about individual proxied TCP connections, only each individual connection gets 'punished'. Courier will tarpit whichever TCP connection is causing errors; other concurrent TCP connections -- even from the same host -- are unaffected.

However there are other negative reasons for this setup. One of the available defenses is an overall per-IP address (or /24 netblock) connection limit. This normally prevents a hostile attacker from flooding your server with thousands of connections and keeping it from accepting mail from anyone else. This works hand in hand with tarpitting; a hostile attacker is confined to a limited number of connections, all others in excess are dropped, and the remaining connections are tarpitted at the first sign of trouble.

connections originate from the same IP address as far as Courier is concerned, and there is no way to discriminate between different sending IP addresses -- and you are vulnerable to being bombed, unless your proxy has the ability to restrict the maximum number of open connections from the same source that it will forward.