Late to the answer here, you could roll your own, but my favorite is MailChimp for a smaller subscriber base. Plus, they put out a good vibe as a company. I used to want badly to work for them, but I couldn't because I didn't live in Atlanta, and because I was in a non-compete since I worked in the same space. sigh. Email service providers come in different flavors, depending on the size of your lists. When you get into the millions of users, you'll need to come up with a different strategy. I used to work for an email service provider who served emails for the likes of Costco, MSNBC, and Alaska Airlines. Sending emails en masse is a whole lot trickier than it should be, sadly. The mechanics are simple enough, but when you get into having to get thousands or millions out the door in a short period of time, the service has to be built to handle that sort of scale. If you're just starting out your subscriber base, that's not something you need to worry about at this point. But, if you intend to rapidly grow, then you should absolutely keep this in the back of your mind.
Edited to add: if you are going to go with a service, make sure the service doesn't treat transactional emails (the one-off, one-by-one, event-driven type emails) as a second-class citizen. Most services have APIs, but some don't do transactional, only emails to lists.