E-Mail Headers: Where the Action Is

"As e-mail marketers, we spend a majority of our time creating e-mail bodies. However, the most important part of a message may be something outside the creative process: the e-mail header, those generally hidden lines of code at the top of each message. Now that the e-mail industry is using authentication, reputation, and accreditation, the e-mail header plays a critical role in an ISP's decision to block or deliver a message. Most of us, though, let our IT departments or e-mail service providers (ESPs) worry about the header. With our basic walkthrough, you can interpret what the header tells you about message delivery. We'll also provide a couple reasons fiddling with some list software settings can actually hurt deliverability." Source: clickz.com