Retailers sent 55% more emails on Black Friday than they did in 2015 and 42% more on Cyber Monday.

Consumers’ inboxes this holiday season are a lot fuller than they were a year ago.

Retailers sent 12,606 email campaigns on Black Friday, which was 55.4% more than they sent in 2015, and they sent 10,334 email campaigns on Cyber Monday, a 42.0% jump from a year earlier, according to data released today by digital marketing vendor eDataSource, which gathers email marketing data from a panel of about 1.4 million consumers around the globe.

The email surge didn’t just center on those specific retail events. Retailers sent 38,268 Black Friday-themed campaigns during the 14-day period that began 10 days before Thanksgiving and extended to Cyber Monday, which was about 41.9% more than the 26,969 emails they sent a year earlier. They also sent 26,969 Cyber Monday-themed emails, a 28.2% jump from 10,938 emails they sent a year earlier.

There’s good reason those volumes are rising: Consumers are looking at those emails. About 20% of Black Friday-themed emails had a read rate exceeding 20%, up from 13% a year earlier. Roughly 19% of Cyber Monday-themed emails had a read rate over 20%, up from 12% in 2015.

It isn’t just that shoppers are looking at those emails, they’re also clicking through and buying from them. For instance, Adobe Digital Insights says email drove 18.1% of online sales from Nov. 1-28, behind only search (38.5%) and direct traffic (25.3%), and ahead of referral sites (16.0%), display (1.2%) and social media (0.9%).

advertisement Inc., No. 1 in the Internet Retailer 2016 Top 500 Guide, sent more Black Friday- and Cyber Monday-themed emails than other merchant—by far. Amazon sent 271 Black Friday-themed emails during the 14-day span, up 40.4% from 193 a year earlier. It also sent 148 Cyber Monday-themed emails, up 26.5% from 117 in 2015. That reflects Amazon’s approach in which it carves up its large customer base into discrete segments based on what customers have bought and looked at on its sites and apps, says John Landsman, eDataSource’s director of strategy.

Amazon is as sophisticated a one-to-one marketer as I’ve seen during my 30-plus years in database marketing,” he says. Amazon’s massive scale enables it harness a vast amount of information it knows about its customers to finely tailor its messages to shoppers, he adds.

While Amazon’s email volumes grew, Wal-Mart Stores Inc. (No. 4) was the second-most prolific retailer in terms of email volumes and  actually sent fewer themed emails this year. It sent 85 Black Friday-themed emails, a 12.4% dip from 97 a year ago and just 14 Cyber Monday-themed emails, a 68.2% decline from 44 a year earlier. In part that reflects a strategic shift to more holiday-themed promotions rather than those focused around the major sales events, Landsman says.

Looking at the top-performing Black Friday and Cyber Monday emails it’s clear that consumers were drawn to aggressively promotional subject lines tailored to particular customer segments. Those messages often offered “Black Friday” or “Cyber Monday” deals well ahead of the specific day. For instance, J. Crew Group Inc. had the top-performing email in terms of read rate—which refers to an email being opened for at least 8 seconds—with 46.7% of recipients opening up its Nov. 18 email that featured the subject line “For Cardmembers: Early access to our biggest Black Friday sale ever.” Similarly, Sephora USA Inc.’s Nov. 27 email with the subject line “Today is Sunday, but for you it’s Cyber Monday” was the third-most read email with a 38.2% read rate.

While those results suggest a strategy that retailers may want to pursue, they should be careful in doing so, Landsman says. “The holiday season presents special opportunities and risks,” he says. “Every retailer is under extremely heavy competitive bombardment and driving maximum impressions may be more important than targeting virtuosity.”