Spamassassin has had a bug for a while, marking any mail from 2010 and later as spam because it’s from “far into the future”. This was very crudely done as this regexp: /20[1-9][0-9]/. Because of that, almost all mail from 2010 onward is marked as spam.
I Changed the regex to match for 2020 or later, but that’s not really a fix. Even the spamassassin maintainers ‘fixed’ it that way.
What I have to look out for though, is that this file may get overwritten when I upgrade zimbra. sa-update doesn’t seem to work on zimbra, so I don’t really know what the best way of getting new rules is.