Cyber Monday has been the most popular day to send retail emails for the past two years. Last year, 70% of the top online retailers tracked by my Retail Email Blog sent at least one promotional email on Cyber Monday. That was up from 68% in 2007.

The last time that Cyber Monday wasn't most popular retail email day of the year was 2006, before the day was a big deal. That year, Cyber Monday was the sixth most popular day to send retail emails; the day after Christmas was No. 1.

Since then, Cyber Monday has gone mainstream, thanks to lots of media coverage, which has in turn given retailers more confidence in pushing Cyber Monday promotions. In fact, the day has become so well-known that retailers promoted it by name in their email messaging last year -- twice as much as they did in 2007.

Retailers are also more comfortable promoting Cyber Monday by name than they are Black Friday. While 36% of retailers referenced "Cyber Monday" in their emails on Dec. 1 last year, only 25% of them referenced "Black Friday" in their Nov. 28 emails. For a relatively new term, that's a pretty big endorsement by the nation's largest online retailers.

Retailers' increasing comfort level with Cyber Monday has helped the day become responsible for more and more retail sales. According to comScore, Cyber Monday accounted for $846 million in online spending last year, up 15% from 2007 -- and the second-highest amount spent in online retail since the $881 million spent on Green Monday 2007.

Given Cyber Monday's popularity with email marketers -- and since up to 40% of online retail sales can be driven by email marketing -- it's hard to deny that email marketing is the engine behind Cyber Monday's rise to prominence. While it hasn't yet lived up to its branding as "the biggest online shopping day of the year," it may get there this year. If it does, email marketing will be the one to thank.