A third party shouldn't market your products better than you.

The scope of crossover that Amazon's emails has with other retailers is almost difficult to comprehend.

That's why marketers pay such close attention to the ecommerce giant's messaging; or at least they should.

But there's another digital brand that retailers should also keep a close eye on when it comes to email marketing: Groupon.

We always talk about Amazon because the overlaps are so large, and so widespread.

But it occurred to us in looking at recent analysis that Groupon was also a very considerable presence in a lot of their customers' inboxes, says John Landsman, director of strategy and analytics at eDataSource, and author of Spotlight on Groupon's Email Marketing, a recent study conducted by the email marketing consulting firm.

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