This subject is worthy of a much larger article, but for now, in brief, here are the basics.
This is part of marketing, so that more people are aware of your brand, what services you offer, and what you do. The more people who know about your business and see you as an expert, the more likely you are to be referred to other dog owners by your clients, but also by people who may never have used you, but appreciate your expertise through your newsletter.
They work best when they have categories. So your categories could be:
The number one reason people sign up for email newsletters is to get offers and discounts, so within a week of your event, send an email out to all your new members. Remind them they signed up to your newsletter, go over all your services, send them an offer or discount on services (all offers should have an expiry date), and let them know they will be included on your newsletter, and let them know how often this will be sent.