Postmark user over here as well.
I operate a WordPress hosting company, and have all servers, and WordPress itself set to send emails with them, using their SMTP servers with Postfix on the servers, and their API with WordPress. Never had any issues.
Another option is Mailgun, which I used for other projects in the past. While I did not have any issue with them, if you look around, you might see that the experience is not shared by everyone.
Postmark offers faster delivery times, and better deliverability in general, but it is not like Mailgun is terrible at that. With Mailgun you spend less money, which might be something to consider depending on your sending volume — little differences in price pile up fast.
On the other hand, Mailgun has been involved in data[1] leaks[2] before.
Personally, I value quality support — Postmark has an excellent support team —, and delivery times a lot more than price, which is why I chose Postmark, but it is great to have options.
[1] https://www.mailgun.com/blog/mailgun-security-incident-and-i...
[2] https://www.zdnet.com/article/mailgun-hacked-part-of-massive...