If you wish to do an email blast to all your subscribers about something very specific - such as newsletter updates, blog feeds, etc., then you can use broadcasts on Gist.
Let's look at a use case where you'd be using the broadcast feature to send a one-off email to your newsletter subscribers notifying them of a new post on your blog.
The content of a newsletter notification email:
What you say in your first email to your prospective customers is of great importance, because that message helps you build a personal relationship with your users. Even though broadcast emails are one-off emails that you send from time to time, it is important that you send a friendly, warm welcome message to make your users feel at home.
Greet your customers by using their first name, instead of a generic hello. Gist adds a first name variable by default. Keep the tone friendly and informal. Encourage users to take action on your site.
Here's an example of a good broadcast email that you can send to notify your subscribers of a new blog post.
When to send broadcasts?
Since broadcasts are one-off emails that are centered around a certain event, it would serve you best if you time the broadcast right. In this case, since you're notifying your users about the blog post that you just published, it would be good to send the broadcast email right after you publish the post on your blog.