More specifically, the 91.4% of the authenticated non-spam emails sent to Gmail users come from senders that have adopted one or more of the following email authentication standards: DKIM (DomainKey Identified Email) or SPF (Sender Policy Framework).Google Says 91.4pct of Authenticated Non-Spam Emails Via Gmail Use DKIM or SPF Standards [googleonlinesecurity.blogspot.com]

Here are some statistics that illustrate the scale of what we’re seeing:

76.9% of the emails we received are signed according to the (DKIM) standard. Over half a million domains (weekly active) have adopted this standard.
89.1% of incoming email we receive comes from SMTP servers that are authenticated using the SPF standard. Over 3.5 million domains (weekly active) have adopted the SPF standard.
74.7% of incoming email we receive is protected by both the DKIM and SPF standards.
Over 80,000 domains have deployed domain-wide policies that allow us to reject hundreds of millions of unauthenticated emails every week via the DMARC standard.