A decade ago I went to my country's embassy to renew my passport, and they now use my email to subscribe me to the newsletters of any new political party. All unsubscribe links just 404s. Shameful behaviour.
Anything I receive from any of their political candidates goes straight to spam now. The hope is that I am training the spam filter so it marks those as spam for all other users as well.
It's simple really: have clearly visible, working unsubscribe link in the body of the email that doesn't require jumping through hoops, and be from a company I know and use. Otherwise the spam filter learns about it.