Looks like they're using AWS SES. From personal experience, none of them (SES, Mailgun, Sengrid) are suited for critical, transactional emails. I'm working in the same space and we're running our own backend.
@ctas since you're running your own backend, how did you solve the reputation problem to ensure good deliverability? I would have thought that using a dedicated IP on an existing provider like Sendgrid would be the best solution for critical transactional email.
There's no one-size-fits-all-solution to ensuring good deliverability. Dedicated IPs are often (not always) a sign of weakness as they indicate that the shared IP pool lacks good reputation.
We run an exclusive transactional email service, meaning customers go through an approval process before they can send live emails. Transactional emails by definition are sent as a reaction to a users' action and _should_ therefore have a low spam-rate and fast time-to-open. If you can ensure that your customers are good email citizens and actually send only transactional emails, you don't even need to upsell dedicated IPs to your customers, as the deliverability of a high-quality, shared IP pool is better than that of an isolated IP address.
I suspect most serious customers will want dedicated IPs, and then they can send using their own servers. Relying on shared IPs is always going to cause problems, especially for high volume users that can sustain enough traffic for their own clean IP sets.
I ask because it takes a long time to build up trust reputation among the major email providers to help ensure deliverability.