Mailgun

Mailgun started in 2010 as a Y Combinator company built by developers who wanted the email equivalent of Stripe: an API you could integrate in an afternoon instead of a week. Rackspace bought it in 2012, ran it as part of its cloud portfolio for almost a decade, and Sinch acquired the whole operation in 2021, which is also how Mailgun ended up as a corporate sibling to Mailjet under the same parent despite the two products barely overlapping in their audience.

It's a non-AI service through and through. What set it apart early on, and still does, is how much thought went into inbound email, not just outbound. You can route incoming mail to a webhook and parse it programmatically, which sounds niche until you're building something like a support ticketing system or an email-to-app workflow, at which point it becomes the reason you picked Mailgun over anyone else. The validation API for checking addresses before you send to them is solid too. It's not the cheapest option on this list, and the free trial is short rather than an ongoing plan, but the documentation is still some of the clearest in the category, and that counts for a lot when you're the one debugging a delivery problem at midnight.