DKIM

I found that Google/Gmail was suddenly filing email from my domains into the spam folder of the recipients. At that point I had implemented SPF a few years back. It's difficult to access whether SPF pass increases the chances for successful delivery, but SPF fails definitely increase the odds of rejections, which is a good thing, because it makes my domains unattractive for spoofing. This in turn has reduced the number of non-delivery messages (back-scatter) for spam that claimed to originate from one of my domains to just about none.