Many of our email campaigns turned out to not need extensive customisations, which is why we started off doing it in-house. I'd much rather let the business folks be responsible for turning off/no campaigns than having to wait for me. Plus that darn chimp is cute!
Now onto the technical stuff.
One of the main reasons that it is impossible to compare Postmark with Mailchimp (and even difficult to compare it to Sendgrid or SES) is our laser focus on transactional email delivery.
We've been in the email business long before building Postmark, and we know the differences between how ISPs treat transactional (event or customer triggered) emails and bulk (marketing) emails.
We also know that because ISPs treat these emails differently, they need to be SENT differently. This is why we strictly prohibit bulk sending on Postmark, to make sure that the transactional email sent through our system has the highest rates of (and the fastest) delivery possible.
We know that we've had customers leave because we limit sending to transactional emails - but its to the benefit of the customers we have because we maintain extremely high delivery rates.
Email delivery is, as we've leanred, very susceptible to "Tragedy of the Commons" in the fact that it's very easy for a small number of people to spoil the value for the many. We're protective of our infrastructure this way because our customers can trust the result.
Comparing Postmark to SES on the pricing front is something we chose to avoid as well. You can drink well whiskey or top shelf bourbon. Both can get you drunk, but you'll only really enjoy sipping one of them :)
