I'm hopeful that the product remains the same. I chose Postmark because it was the most simple transactional email provider I could find.
ActiveCampaign on the other hand was always a nightmare for me at my prior company. Their API had a 5 request / second rate limit. Which made it almost impossible to use for 50,000+ customers (syncing up emails / tags / campaigns).
I worked on a project that used the ActiveCampaign API. It had tens of thousands of users and an elaborate tagging system.
There were operations that we'd have liked to be synchronous (in the browser), but the rate limit and no useful batching mechanism (AFAIK at that time) meant they took over a minute to complete.
ActiveCampaign on the other hand was always a nightmare for me at my prior company. Their API had a 5 request / second rate limit. Which made it almost impossible to use for 50,000+ customers (syncing up emails / tags / campaigns).