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.
Email is in my bio, if you have other questions.
What's this service called? A link also works. Imagine many people in this thread are looking for alternative transactional email services...