Display system status
Let's start with the sign up process itself. A basic attribute of any usable software product or website is its ability to display system status accurately. In other words, it should tell you what's happening when you ask it to do something. Usable products don't leave the user guessing about whether some process failed behind the scenes. Yet dozens of large companies don't clearly communicate what just happened when you opt into their email campaigns. In such cases, the user has no way to know if the sign up process was successful, so they sometimes repeat the process, compounding the mistake.
Enable user control and freedom
This is another "ancient" usability principle (or heuristic) that was first used to evaluate desktop software, but also holds true for the emails your company sends. Are your users in control? Do you tell them how many emails they will receive if they subscribe and give them some way to manage this? Can they immediately unsubscribe, or do you put them through annoying confirmation loops? Can they easily scan to the content they value and skip the rest? Can they freely reply to your messages by hitting the reply button, or do they have to take extra steps to contact you on your terms.
Respect the context of use
Email marketers have historically been too insensitive of their recipients' real context of use. What's the context of use for receiving an email in 2008? In most cases, it's about rushing to dig through a mountain of spam. Jupiter Research recently projected that spending on email will surpass $2 billion by 2012, so any single message is just a tiny part of that growing mountain. Yet many marketers continue to focus on creating fancy business rules based on trend analysis, pumping out millions of increasingly irrelevant messages. A few hours of interviews with real life users might just bring a fresh blast of reality into their planning processes
Amazon.com's subject line gets the story across even if you're too busy to open the actual message.