On one hand, it's awesome Wildbit was able to sell Postmark on their terms. To be able to build something, make money, and then walk away when you want is a huge accomplishment. Hats off to them.
On the other hand, the reason I always chose Postmark over Sendgrid, MailChimp, et cetera, even though they were much smaller, is because they seriously cared about their customers. Support is fantastic of course, but it was really the little things that made the experience great.
An example, they manually vetted each of their API consumers to ensure one bad apple doesn't spoil the bushel. It's probably not cost effective to gate customers like this, but in the end I don't have to worry about MY email not being delivered by one of the major providers because some other person used Postmark to send a spam campaign. Stuff like this happened all the time with Sendgrid and the only remediation I've ever seen was "upgrade to our $90/mo plan and manage it on your own."
Maybe ActiveCampaign will keep this level of quality and care (it is the same team after all), but I can't help but be a little cautious. I've been burned one too many times by an acquisition of a great product by a not-so-great company.