From this Quora answer [2]:
> Apparently, only one partner, Robert Morris, reviewed the SendGrid application, and he gave it a highly negative score, calling it a "spam company." That pushed it so far down the application queue that no other partner reviewed it. Paul talks about how they changed the application process after that so that any applications negatively scored by Robert would also get reviewed by Paul.
(Interestingly, venture returns skew so highly to big winners that SendGrid's objectively huge outcome would not even have had a material impact on YC's current portfolio value of nearly $1T [3]. I'm not aware of any current $10B+ companies that were rejected by YC, though there may be some.)
[1] https://www.zdnet.com/article/twilio-to-acquire-email-api-pl...
[2] https://www.quora.com/Why-was-SendGrid-rejected-from-YCombin...