The huge numbers of deals and newsletters companies send via email may be sparking a backlash. For the past several years, a little over 80 percent of emails have made it to users' inboxes. However, in the second half of 2011, researchers at email certification company Return Path saw a sudden 6 percent drop in the amount being delivered worldwide. A recent report found that only about 76 percent of messages were reaching the inbox by the end of the year, marking the first "major decline" the company says it has ever seen. While a number of messages were heading to the spam folder, an increasing number were being blocked by ISP-level filters — 15 percent, or 20 percent more than were being blocked in the first half of 2011.

So why are these filters blocking more messages? It may be because of what Return Path calls "email overload." Rates of "true spam," the company says, are low, but a Microsoft study estimated that half the average user's inbox was filled with newsletters or deal emails. Although users have often signed up to receive them, the increased volume around holiday sales in particular leads ISP-level filters to adapt and block bulk mailings. The users, too, aren't so happy about their crowded inboxes: Return Path says that "people have opted into so much email that it has become a challenge for them to deal with it."

Ultimately, Return Path's mission is to get as many promotional emails as possible sent successfully, so its recommendations aren't the kind of thing most users want to hear. However, the company admits that "email marketing seems to be a victim of its own success," and that companies need to plan ahead if they want their messages to be seen. Hopefully that will include limiting the amount of emails to a volume that the average customer can handle.