Mandrill

Mandrill launched in 2012 as Mailchimp's answer to the transactional email gap in its own product: great for newsletters, useless for password resets and receipts. For a few years it worked as a mostly independent add-on with its own free tier, until Mailchimp announced in 2016 that Mandrill would only be available bundled with a paid Mailchimp plan. Developers who'd built entire side projects on Mandrill's free tier were given weeks to migrate, and the backlash was loud enough that people still call it the Mandrillpocalypse nearly a decade later.

Mailchimp itself became part of Intuit in 2021, and Mandrill, a non-AI product like the rest of the transactional field, has continued on largely unchanged since the 2016 shift. It still requires an active paid Mailchimp account, priced on top of whatever marketing plan you're already paying for, with transactional email sold in blocks of sending volume. That makes it a strange recommendation on its own, since nobody signs up for Mandrill in isolation the way they might sign up for Postmark or SendGrid. It only really makes sense if you're already deep into Mailchimp for your marketing email and don't want to manage a second vendor relationship for transactional sends, in which case it's a reasonably convenient bolt-on rather than a product you'd seek out independently.