Postmark is the best reputation in transactional email, which makes it the worst tool for cold email. The problem is not deliverability, it is design and policy.
Postmark will not send marketing campaigns. Its platform is built around one assumption: the recipient expects this mail, because they signed up, bought something, or triggered it. Cold email violates that assumption by definition. The streams model, the content rules, and the reputation policy all assume transactional mail, and cold outreach does not fit the model.
Postmark's acceptable use policy explicitly restricts unsolicited and bulk email. Running a cold outreach campaign on Postmark is a terms violation, and Postmark enforces its policy more strictly than most providers, precisely because its reputation depends on it. The risk is not just that your cold mail lands in spam; it is that the account gets suspended and your transactional mail, the mail that actually matters, stops sending.
Even setting the terms aside, Postmark's value is its clean IP reputation. Cold email is the fastest way to damage a clean reputation: unknown senders, no prior engagement, complaint-prone recipients. You would be trading the best asset in the product for a use case the product does not support.
Keep Postmark for the transactional mail it is good at. For cold email, use a separate sending domain, a provider that explicitly permits outreach (a dedicated cold-email tool or a transactional provider with a dedicated IP and the right terms), proper SPF, DKIM and DMARC on that domain, and a two to four week warmup starting at 10 to 20 emails a day. The two systems never share a domain, so the cold-email reputation risk never touches your transactional mail.
Whichever provider you pick, the setup on your domain decides the outcome.
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