Microsoft recently announced that on November 1, 2016, they will stop generating updates for Microsoft Exchange's "SmartScreen" spam filters used in Microsoft Exchange Server and the Outlook (Windows) desktop client. Read more about it here.
What does this mean? This is probably a good thing. These were primarily content-related filters and content filtering isn't really "where it's at" when it comes to best practices with regard to spam filtering nowadays. I believe that this ultimately will drive users to newer solutions that are probably going to be more focused on sending reputation, meaning that us deliverability and email technology-related folks will eventually no longer have to deal with Outlook desktop client spam folder issues, which were often a confusing outlier when reviewing email deliverability results.
Click here to read more on this topic, from Laura Atkins of Word to the Wise.
If you're having deliverability issues, I can tell you from my experience that you aren't really going to be able to get back to the inbox if any of the following types of subscriber list sources are in play: