or do I just need to leave it turned off?
Google protect all Gmail accounts [s]using Postini [/s]spam/virus filter > Used

Postini > https://en.m.wikipedia.org/wiki/Postini

2012 > Google Postini Email services will be shutting down, new encryption service appears to be on the way
https://9to5google.com/2012/08/21/google-postini-email-services-will-be-shutting-down-new-encryption-service-appears-to-be-on-the-way/

2015 > Google starts applying artificial intelligence to Gmail spam filters
http://www.digitaltrends.com/android/google-starts-applying-artificial-intelligence-to-gmail-spam-filters/

http://www.pcworld.com/article/2995524/security/google-yahoo-tighten-spam-filtering.html

http://www.techlicious.com/blog/gmail-spam-filter-artificial-neural-network/