People who have subscribed to your email newsletter are the most valuable group: people who actively want to hear from you about your products or services. As a content publisher--you have a huge privilege. When done correctly it is relevant, personal and welcomed.
First, always make sure you have the recipient's permission to email them. I've had the experience on social media where I subscribed to an email subscription and I received an ebook in exchange. The website owner first emailed me, then I had to respond to his email by clicking on a link to verify that it was my email address and that I was giving him permission to use my email.
Getting permission to use a person's email is important because people take their email inboxes personally--almost fanatically, and unsolicited email that arrives in that personal space can be offensive as any personal intrusion. So unless you have someone's explicit permission to email them, don't do it. It is not worth it.
Secondly, set clear expectations. If you are going to be emailing monthly--say so. If you are going to be emailing different types of communication (newsletter or product promotion newsletter) let people know and preferably give them the option to subscribe/unsubscribe to the different types of promotions.
Why so much caution? You do not want to be labelled a spammer!
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