SendGrid

I use it for my SaaS company where we send transactional e-mails with Sendgrid.

effective for marketing emails

Easy to use and hook up, love that you can get a dedicated server for cheap. Analytics leave a lot to be desired as they paywall you for specific user activity beyond 7 days. Pretty affordable.

Easy to use, with good monitoring options and great support for experienced and beginner developers. The free tier is also a great start for smaller projects.

I use SendGrid for powering my newsletter. I love how simple they make the commands and the user interface is very intuitive. My only issue is with the API documentation. It is a little stale and leaves you hunting for the correct schema a bit. But once you find it, it works great. I use the dynamic templates, marketing segments, single sends, the API, and analytics. It is fully featured and the free tier is among the best out there.

It stops you from going crazy for email management

We use sendgrid in addition to mainchimp and mailgun. Its just safer to have multiple email vendor integrated so that if there any serious outage, you can fall back to the next one.