oh, fascinating, but don't you fix that by having separate infrastructure? they don't say that explicitly, but they do advertise that they have separate infrastructure for transactional and marketing.
My impression is that their "streams" are just ways of segmenting different types of content within an account, not actually using separate infrastructure to send the actual mail.
hey. notice: i work for mailtrap.io but I have domain knowledge
It's always better to separate your transactional and bulk (marketing traffic). We do it as well as it's also best practices recommended by Gmail (use different IPs, and even subdomains).
Also another thing is suppression list: as somebody marks you bulk email as Spam, they still can receive their e.g. pass reset emails.
It's probably nothing cheaper than SES, but you probably need more time with it. And if you have any issues with deliverability - you are on your own. Postmark or we provide support, UI, etc for extra costs.
Maybe I'm wrong about that, though.