Best would be Campaign Monitor, best-with-free-tier probably Mailchimp.

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.

I was reading my book on Defold development, and I heard about https://sendy.co/ Apparently it's a lot cheaper than say MailChimp.

Mailchimp!

writing such a thing including mail tracking is quite easy :) i worked for emarsys who is one of the bigger ones for professional mail sending :)

usually you maintain the list yourself and just sync it. but i havent looked into the buisness for 5 years :)

in the end, if you don't wanna track your emails services like sendgrid are an possibility or other relays or you can use mailchimp if you need a/b splitting and inbox previews .

it just depends on the languag and the need. if you want to maintain it yourself and want to have multiple subscription options I recommend looking into bitmaps to use easy scalable filters :) and use an server rewrite of an image src for the opening tracking :) so at least you have a starting point of the basics :)

Majority of websites use MailChimp for their newsletters. It's pretty easy to setup and most importantly is free when starting out. There are also some self-hosted options out there like Mailtran, or you can build your own solution from scratch with something like Amazon SES or SparkPost.