From NEWSLETTER USABILITY:
Newsletters Must be Simple
People get a lot of email. They don’t have time to read a lot of text. Newsletters must be designed to facilitate scanning. In our study, only 23% of the newsletters were read thoroughly. The remaining newsletters were skimmed, read partly, or not even opened — a fate that befell 27% of the newsletters.
The only newsletter in the study that was consistently read every time it was received was Dictionary.com’s Word of the Day, which is very short and also has an engaging layout that doesn’t intimidate users with a wall of text.
In subjective comments, users basically said that newsletters are bad if they take too much time or demand too much work of the user. Newsletters are good if they cut down the time it takes users to accomplish something or if they are quick reads that do not feel frivolous.
Users will often avoid signing up for newsletters because they feel crushed by information overload. It is the job of the newsletter publisher to convince users that the newsletter will be simple, useful, and easy to deal with.
A predictable publication frequency that is not too aggressive is usually best, except for newsletters that report breaking news. Not only are users more likely to sign up for newsletters that feel less intimidating, but a regular publication schedule lets users know when to look for the newsletter and reduces the probability that it will be deleted because it is confused with spam.
Writing good subject lines is especially important, both to encourage users to open the newsletter and to distinguish the newsletter from spam. We recommend including some actual content from the individual issue in each subject line, even though it’s a difficult job to write good microcontent within the 50-60 character limit that is imposed by many email services.