If, indeed, daily deal emails are less likely to clutter consumers' inboxes as recent trends suggest they might, flash sales' more urgent messages will be the clear beneficiary as they won’t be drowned out by other emails that consumers are increasingly viewing as graymail.

Timing these flash sale messages, though, is key. Timing among email marketers has been a long-dissected subject with views across the spectrum of what is the 'best time.'  The argument is more clear cut for flash sale emails: these have to be seen within a specific time period, usually a few hours. According to StrongMail's "How Daily Deal Sites Capitalize On Email, a 10-minute window prior to a sale is best to create a sense of urgency and ensures equal access to limited inventory.

Getting it right is important because flash sale emails can significantly outperform other types of offers in campaigns, a separate report by Experian CheetahMail finds. Email is a key traffic driver for advertising flash sales and represents 18% of the referrals to flash sales websites - higher than social (13%) and search (11%) according to Experian Hitwise data. As with the debate over traditional email, there is little agreement with flash sale timing either. In contrast to StrongMail, Experian CheetahMail found that:

• Three-hour flash sales have the best transaction-to-click rates, which are 59% higher, as well as the highest transaction rates (.14%)

• Transaction rates are 23% higher for evening flash sales compared with lunchtime sales, and revenue per email is 30% higher in the evening.