The attack that hit my Gmail inbox was part of an offering that crooks can hire to flood each medium of communication. Googles engineers managed to block a majority of the junk messages after about six hours, but the company declined to talk about what caused the attack to succeed. A week after the attack, I was lurking on a relatively new cybercrime forum when I stumbled across an automated email flood service offered by one of the founders of the forum. The ad for this service included screenshots of a flooded Gmail inbox that looked like the attack.”]
Source: https://krebsonsecurity.com/2012/07/cyberheist-smokescreen-email-phone-sms-floods/