Postmark! Great API, solid performance. Use it for every project.

You can take a look at GoodSender. They're new, but they promise 100k emails per month for free, plus $1 for each additional 100k. However, they require the customers on your list to explicitly give consent, or you must send templated transactional emails.

Cheap ($0.10/1000 emails) + good deliverability = Amazon SES.

I've been using Postmark for many years, and have zero complaints.

Sendgrid, on the other hand, I would avoid like the plague.