Wasn't Sendgrid rejected by Y-Combinator? I guess it can be hard to tell at the early stage.
https://www.bvp.com/portfolio/anti-portfolio
They were pitched & passed on: AirBnb, Apple, Ebay, Facebook, Google, Intel, Intuit, Paypal, Snapchat, Tesla..
Wow, that has got to be hard to swallow now.