Email newsletters seem so easy, but they are actually the easiest way to abuse your customer relationships. If they aren't thoughtful, relevant, and consistently packaged, you'll be exploiting your customers' time and attention. Good newsletters require a dedicated team of people who know what else is going on around the company, as well as best practices for email. Cheap is deadly. If you are sending email newsletters, you should read the entire article.
MonkeyBrains, the MailChimp Blog:
95% of problems come from newsletters, not promotions, 2008-May-8
Promotional campaigns are hard. Usually, it’s the experienced email marketers who do promotional campaigns. People who run e-commerce websites. People who generate promo codes, and build landing pages. ... The skill level and experience is higher, so they’re more likely to be aware of email marketing etiquette and spam laws and ISP feedback loops. Newbies don’t do promotional campaigns so much. They just do newsletters. Newsletters are easy. Newsletters are fast. And newbies are more likely to make these mistakes (I see it over and over and over again)...