Semi-related: If you are in the market for an email provider for transactional emails, I encourage you to use Postmark. It's a developer focused company with a simple pricing model and great documentation, and their IPs tend to have great reputation scores. I don't work for them, but I switched from one of these two and am very happy I did.
Also their deliverability is excellent and when there are deliverability issues, it's easy to find exactly why. The logs also surface the info we need to prove what actually happened during a nondelivery event. ("Here is the actual log line from your MX server rejecting the mail we sent.")
For small near-zero-budget projects, I still use Amazon SES. It's Good Enough for most uses and nobody else with even decent deliverability can touch them on price. All my hobby projects combined are billing well under $1/mo for SES usage.
SES is also decent for some very specific inbound email use cases. If you need to receive email from some system that doesn't have a real webhooks type outbound API, but does have robust email notifications, and send the body to a Lambda function that parses it and makes the API call that system ought to have just made in the first place.. SES is perfect. (And unfortunately all the major systems I work with that process inbound email are stuck needing an IMAP connection - if I was able to use an API based inbound email receipt scheme, and needed to receive mail from various domains, I'd consider Postmark for the better diagnostics and logging.)