avoiding overload on port 587

[Tomas Macek]

I don't understand now, how Postfix behaves when listenting on 
submission port 587.
Our mailserver is sometimes overloaded on port 25, so we want to use 
postscreen. But I don't understand, how Postfix works when it's stressed 
on port 587, when spammers connect to that opened port and want send their 
"emails". In document http://www.postfix.org/STRESS_README.html there 
is:

NOTE: To avoid "overload" delays for end-user mail clients, enable the 
"submission" service entry in master.cf (present since Postfix 2.1), and 
tell users to connect to this instead of the public SMTP service.

Should this mean, that Postfix by default does not use counters like 
smtpd_hard_error_limit, smtpd_junk_command_limit and maybe others on 
sumission port? On 
this port I would prefer using some kind of smtp auth and this port should 
be world accessible to allow the clients using other networks to 
authenticate and send emails.

Regards, Tomas