Hi, I'm the product manager on Postmark, and wanted to chime in with a bit more of our rationale for mostly using shared IPs. With our exclusive focus on transactional email and high deliverability, an existing reputation is extremely important. By using shared IPs customers can leverage our reputation, which we police and protect heavily. And since transactional email has better engagement overall, it increases our deliverability even more. Most senders don't need dedicated IPs since an IP with no reputation is worse, so we believe it's better for the majority of our customers and their deliverability to use shared IPs.

That said, there is a case to be made for very large senders to have their own IPs. We agree with that. Our point is mainly that the vast majority of senders don't need it, and should rather use the stellar reputation of our shared IPs to ensure good deliverability.

If anyone would like to chat more, I'm happy to jump on a call - https://calendly.com/rianvdm/postmark-customer-calls/