I have built a website and within 1 month I've got like 400 verfied users in my website. While signing up they have agreed to the term that I can send them emails. So I'd like to send them email to make them to return again. But the problem is that my ip is a "new ip" and I don't have enough reputation. so if few people mark my emails as spam then all my emails will also go in spam folder. So I think the best choice is to use some mail service.