Two technologies are used in conjunction: DKIM (Domain Keys Identified Mail) and SPF (Sender Permitted From).

FORBES: Authenticate This!

Google Apps customers have their own domains, which places the onus for setting up DKIM on them.

FORBES: Google Says Google Apps Domains Were Protected From Massive Spoofing Vulnerability

Hopefully, this post will help raise awareness, particularly among smaller companies, of the advantages of adding DKIM authentication.

FORBES: Google Says Google Apps Domains Were Protected From Massive Spoofing Vulnerability

The protocols for that kind of authentication, known as DomainKeys Identified Mail (DKIM) and Sender Policy Framework (SPF) are hardly new.

FORBES: Cisco-Spawned Startup Agari Aims To Finally Kill Phishing

Like many other companies, Google uses a system called DomainKey Identified Mail (DKIM) to validate that mail sent from Google.com really comes from Google, and not from a scammer pretending to be Google.

FORBES: Google Says Google Apps Domains Were Protected From Massive Spoofing Vulnerability