As for having the SMTP port exposed globally, a simple firewall and/or whitelist should always be enabled, which prevents the smtp being a publicly accessible port.
Guess you misunderstood the case. It's not about the exposed port, it's about the exposed forwarding address which is visible and declared as not reachable in the bounced mail.
One can set it to discard bounce notifications for specific servers as-well as queue the emails indefinitely for a specific smtp server.
Well, assuming there is a bunch of different servers on the market, a blanket statement like this is inappropriate. Not every server has options to define those kind of rules.
BTW: if the milter port wouldn't be accessible, the mail is stored in the milters queue until the port becomes available again, but no NDR rule will be executed.
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