Mailerlite is hands down the best email marketing tool for small business, and most business in general. I've run hundreds of campaigns through Mailerlite and my clients love it. Mailerlite gives you more features on their free plan than any other email marketing software and their paid plans are still incredibly inexpensive. What is also great about Mailerlite is that it's a full suite of tools beyond just email. You can create pop-ups, landing pages, websites, and even take payments through emails you send with a Stripe. If you could only have one tool to invest in for your business, Mailerlite would be that tool.
Mailerlite doesn't just have great products, they have a great company.