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 cus

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.