using postscreen on port 25

I'd like to use postcreen as some kind of spam protection. According to
documentation

* postscreen(8) should not be used on SMTP ports that receive mail
from end-user clients (MUAs). In a typical deployment, postscreen(8) is
used on the "port 25" service, while MUA clients submit mail via the
submission service (port 587) which normally requires client
authentication.

But we have clients, that send mails on both port 25 and 587. I
really cannot use postscreen? I don't
understand why exactly. What will happen if I use it?

Tomas

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