Satisfy the expectations you set at signup. Don’t send more content than you advertise, or you may end up flagged as spam.
Make sure that what you say in your subject line relates to the rest of your mail. Your reader will hate to find out that you sent them an information in the subject line that has nothing to do with the rest of your email.
Keep your content applicable to what reader signed up to receive, and you will reduce the likelihood they will recognize it as spam. Make sure that you segment your lists!