We knew that transactional email was a problem worth solving
Really? Hasn't this been solved already, by many businesses (SendGrid, Mandrill, SES, MailJet) – I'm happy you've managed to build something and are seeing some success, but I don't think you're solving a problem that hasn't yet found a solution.
Altavista worked ok, but I'm glad Google came along.
I think that your comment isn't a bad one, but should go more in depth. Some markets really are unassailable - others aren't. The reasons are many and sometimes complex, so analyzing one is often quite interesting.
I don't think unfunco is suggesting that the market is unassailable. The point seemed to be that this problem wasn't one that needed to be solved (because there were already solutions), not that there isn't room for another solution-provider.