Mounting evidence suggests that brevity is key in E-mail marketing. via OPA Intelligence Report

NN/g: Keep e-newsletters brief, less frequent
Let's keep this simple. Nielsen Norman Group's second email newsletter study brought good news, bad news and some tips for online publishers. Good news: Recipients can tell the difference between spam and opt-in newsletters. Bad news: Recipients are still stressed about inundated inboxes, and have less tolerance for newsletters that waste their time. Tips: People want email newsletters that are informative, convenient, and timely. "General information such as that in human interest stories and columns works best in traditional media," study co-author Jakob Nielsen told Clickz News. "Email newsletters must leverage the benefits of new media -- instant transmission and narrowcasting." Duly noted.
» Study: E-Newsletter Readers Grow Itchy Trigger Fingers (ClickZ News)
» Nielsen offers e-newsletter usability guideline (Publish)
» Study: Email Newsletters Too Long, Too Frequent (MediaPost)
» Email Newsletter Usability (NN/g executive summary)