Email marketing has officially hit its stride, and is taking its rightful place alongside traditional marketing efforts. In its recently published US Interactive Marketing Forecast, Forrester found that many marketers plan to skip direct mail altogether (to the woe of the U.S. Postal Service) and go straight for email. The forecast predicts an 11% growth in email marketing over the next 5 years, and reports that 97% of all marketers say they’ll use email marketing in 2009.

Which makes sense, really — email is cheap, customizable, easy to manage, and more people than ever are accepting of it. Email has become mainstream.

What’s interesting about Forrester’s report, however, is that the research credits the growth of the “social inbox” as one of the reasons for email’s reign. And that’s where things get a little trickier.

For those not familiar with the term (it was new to me), the social inbox bundles a few different concepts. First, there are the true inboxes associated with social media sites such as Facebook and Twitter — and yes, there are companies out there now specializing in email marketing to those places. The social inbox also describes users’ adoption of email software that aggregates updates from all the different feeds to which they subscribe. (Yahoo! launched this kind of new experience with its Mail application in 2009.) But in general the idea is that social media platforms are becoming fair game for email marketing.

The idea of being able to reinforce your email marketing in multiple places may seem attractive to marketers, but we have to understand the implications. The first, of course, is that the social inbox is still taboo as a place to receive direct marketing — even if it’s from a trusted source. The folks at ExactTarget found that while most users found it almost “completely acceptable” to receive promotional messages from companies by email if the user has given permission to do so, they were much less accepting of those tactics through RSS, IM and especially SMS. (See ExactTarget’s presentation for more detail.) Think about what happens when marketing messages start showing up in users’ inboxes all over the Internet — by some counts, we may manage 10 or 20 different inboxes if you count work email, home email, all our voice mail boxes, social media sites, RSS aggregators — unannounced. How fast will users grow frustrated, then become annoyed, then get really livid?

For this future trend to work, we as marketers need to be sensitive to users’ tolerance for e-marketing, understanding more about our customers and how they want to hear from us. This is key, and the discussion about how to do this has most likely only just begun.

But much of the success of the email-social inbox dynamic duo will depend on content. In all the bytes of communications blipping into users’ points of contact with the Internet, are we giving them anything they really want or that truly helps them in their business and their lives? Are we saying the same thing over and over as we reach out to them through different platforms, or worse, are we saying drastically different things that confuse them or turn them off? Or are we telling a cohesive story, using each outreach as a valuable opportunity to share one more enticing piece of the puzzle and tempt them with information they crave?

The collaboration of email and social inbox has the potential to be a powerful tool — but like with most superpowers, there comes great responsibility.