Anyone who has done email marketing has agonized over the question of how often they should be sending emails. People do want to receive them—in a survey by MarketingSherpa, 60% of consumers said they wanted to receive updates and promotions via email, the most preferred method. But if you send too many, your customers might get annoyed and stop reading them or unsubscribe. Send too few and you could lose revenue and your customers’ attention. So what’s the right number?