[Mailman-Users] Best way to slow down all the spam to my lists?

[Chromatest J. Pantsmaker]

I'm the system admin (though I'm not great at it).  I have a problem with
spam.  Hundreds of spam messages are posted to my lists each week.  They're
non-subscribers so they don't go to the lists, but they *do* go to me, the
list owner which floods my inbox.

I'm looking at the page on how to use SpamAssassin:
https://wiki.list.org/DOC/4.23%20How%20do%20I%20use%20SpamAssassin%20with%20Mailman%3F?action=show


Is that the best way to do what I need?
If so, what's the best method to use in my case?
My system:
Mailman 2.1.20
Apache2 / 2.4.18
Postfix 3.1.0
Ubuntu 16.04.1 LTS