One important decision argument against AWS SES is their policy to keep bounce rate below 5% (account put under review, if unresolevd until end of month, will be suspended, with hard limit of 10%) [1] compared to least strict Postmark's bounce rate of 10% [2].
Sometimes for SAAS products with a huge userbase or freemium pricing model is super difficult to keep the bounce rate so low for transactional emails.
Policy like this is meant to protect sender reputation, not just provider IP pool. High hard bounce rate lowers your inboxing in the long term. If your bounce rate is consistently above 5% consider verifying all new emails before sending them an email > https://www.bigmailer.io/blog/email-verification-service-pro...
Natural email decay rate is 2-3% per month so the goal is to stay close to this range, but the lower the better.