There is an option, i have seen in Fortimail, that makes you able to filter/drop all newsletter emails. I don't understand how Fortimail detects these type of emails. And I am keen to know whether someone has any solution for this on Postfix mail server.
SnapOverflow
This is not something that will be part of the Postfix system itself, although it is almost certain there are plugins that will help.
A commonly used plugin is spamassassin, which parses email and provides a score based on various criteria. There is nothing to stop you adding a plugin to drop newsletter emails. The effectiveness of spamassassin in this instance really depends on a lot of variables, including the number of users, the rules you make/use and what you consider a newsletter. (It could be as simple as tagging anything with "unsubscribe" in the body of the email, but that may be to general or not general enough. Spamassassin gives you A LOT of control, but is non-trivial to use).
One advantage fortinet appears to have is the ability to do heuristics across a wide population of email across many servers which you may not be able to do yourself. In other words, you may be able to craft a 95% solution while fortinet may be able to get a 99.5% solution (figures plucked out of thin air)