When a company first implements email marketing strategies, the first thing that comes to their mind are email promotions. Basically, when somebody buys your product or service, you tend to put something like "tick here to be added to our mailing list".
Many companies, once they obtain permission from their clients, they will start only sending promotional emails to their clients, and they forget about the informative emails. When you send a promotion your are not really building a long term relationship (which is what email marketing is about). While promotions are geared towards making a quick sale, an Informative Newsletter is usually design to provide your clients with information, which they will value, and get attached to your company. Make sure that when you send a promotional email, your clients have opted in to receive your emails, and not only to receive your email newsletters. Because chances are they will probably feel tricked.
It is really important that you know the difference between signing up for a promotional email, or signing up for a email newsletter, in which you are expecting to receive information on the industry that you signed up for or the company. Make sure you send your clients what you wish to receive, and do not offend them by sending the promotions that they do not wish to receive.