Yeah, so I believe Postfix mostly rejects mail if the sender is badly configured. For example, if someone is trying to send you mail from a server that is not the domain they claim they are in the message, I believe Postfix will throw it in the abyss.
However, if your mail is being transported correctly (or acceptable by your Postfix’s configured standards), then it is probably spamassassin, blacklist, or other filter. Obviously, if you’re sending mail from a blacklisted server, it’s going to fail.
If it’s neither a transport issue or a blacklist issue, it probably is Spamassassin detecting the message body as spam, or unwanted by the context or how the email was worded. If the email says something about Russian girls wanting to… then Spamassassin will probably think it is junk. If it is really confident a piece of mail is junk, it won’t even make it to the junk folder. But if you’re getting an email from Google for resetting your password (and it passes through transport security measures from Postfix), Spamassassin probably will think it’s not spam (or a really low spam score). If your friend from 10 years ago emails you asking for money, chances are, Spamassassin will give it a medium spam score and allow it to make it to the junk folder.
So if it makes it through transport, it’s all about the spam score. Really high spam scores don’t even make it to the junk folder. Medium spam scores make it to the junk folder, and lower spam scores make it to your inbox. Spamassassin is always learning too, so it will figure out what you call spam and not.