The first one lands before the sun comes up: "Sale! Our buyers' favorite picks up to 50% off." An hour later, there's another: "4 deals inside and everything ships free!" By the time you get to work, even more: "Exactly What She Wants: Chic Shoes and Handbags on Sale Now!"

In the avalanche of messages hitting inboxes this time of year, retailers' emails work extra hard to stand out, with painstakingly-crafted sales pitches that nudge you through a desired progression that starts with opening the email, reading it, clicking through to the website and, finally, the holy grail, making the purchase.

The average consumer receives about 38 promotional emails each week during the holidays, up from about 25 a week the rest of the year, says Chad White, research director at marketing-software company Responsys . So far this season, Cyber Monday, Nov. 26, was the day retailers sent the most emails, followed by Monday, Dec. 17, the industry's unofficial "free shipping" day. Friday, Dec. 21, is expected to be busy, too.

Despite the glut, email is retailers' most important tool for cranking holiday sales. With 45% of consumers expecting to shop online this holiday season, the Internet is now the No. 2 holiday shopping destination, second only to big-box discount and value stores, according to Deloitte's 2012 holiday survey. Online holiday sales are growing three times as fast as sales overall, 12% versus 4%, the National Retail Federation says.

Stores time the arrival of their emails and design them with attention-grabbing subject lines, personalized messages, chatty advice and animation. They study every aspect of your response, from where and how you opened the email to which words and images made you click.