@robw We are not using 100K/month, so perhaps our experience is towards the smaller end. We were using another vendor (which shall go nameless) for quite a number of years and found their delivery rates poor due to spammers in their client pool. We then wound up spending more for a dedicated IP address to try to combat that issue. Then I found Postmark and our problems disappeared. We will move the last vestige of our email business to their platform in the next release.
As you can tell, we value deliverability (given our smaller volume) above all else, even though there may be lower rates offered by competitors.
FYI, Postmark started off small, but they were acquired by ActiveCampaign about a year ago.
It sounds like you have created much of what Postmark offers using your own SMTP servers and CMS systems. By separating the streams, you have accomplished the same as their "servers" and I can see how integrating your infrastructure with theirs would be redundant and awkward.
An interesting question (in my mind) is whether SendGrid or Amazon SES traffic is treated by inbox providers differently than Postmark. That was the root cause of our migration to Postmark and why I would hesitate to migrate away from them at this point. Anyone with experience or perspective on this?