Industry groups have been working on authentication standards to prevent email impersonation since 2004. Standards for sending and receiving domains help authenticate emails have come from the right sender.
In a blog post, Google has stated that the widespread adoption of these standards has dramatically reduced the ability of spammers to impersonate trusted domains. Over 91% of all non-spam messages sent via Gmail is sent by authenticated senders who are using the standards.
Source: Google blog.
