It is easy to assume that just because people agreed to be on your mailing list, they will read your messages. You are not sending spam and they requested your material, after all.
Aaaah - if only this could be true...
Not in your dreams!
There are many factors determining when the message is read and when it is not. The content, of course, and the quality of your writing style has a *lot* to do with it. But no matter how good the quality of your message is, the look is equally important. Your message must be *easy* and *inviting* to read, otherwise you might lose the competition for the time and attention of your recipient.
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