Here's an interesting post from Jordan McCollum at Marketing Pilgrim regarding the effectiveness of email marketing and how that effectiveness will decline over the next five years:

Forrester Research says that marketers say they see ROI on email marketing that’s two to three times higher than any other form of direct marketing. 66% of marketers agree that email is the most cost-effective marketing tool at their company.

So why shouldn’t you waste money on email marketing?

Because wasting money is bad—and why waste money when you can make sure your email marketing is even more cost effective?

The Forrester study takes a look at the email marketing forecast for the next five years, and there’s good news: email will continue to grow in popularity among users and marketers alike. Total spending on email marketing will soar to $2B in 2014 (up from $1.2B this year):

Don’t Waste Money on Email Marketing