One day at the front page of HN of a single page where the only thing you can do is sign up to receive emails is enough to justify 1,000 emails a day IMO. Without knowing anything else, I think the customer support person that took the time to reply me a few times to explain the issue, could have made the decision that likelihood of my service being spam is ~0%.

edit: that's exactly what they did at Mailgun btw. The first reply came with a "I'll need to check with other team". Hours later, without any followup from me, came the second reply with "Thanks for providing the information. We have removed the limitations to your account." (copied pasted). Excellent execution in both getting the account and keeping the risk of delivering spam through their service very low. Compare this to Sendgrid, with three or four cryptic replies, and a lost account.

Check Amplitude's CEO HN thread about what drove the success of the company to the IPO. Among other things, making it easy/free for small companies to use Amplitude, even though their sole business model was to aim at big corporate accounts. For a few small companies that would turn into big corporate accounts or that were acquired by big corporations that would then adopt Amplitude and become big corporate accounts.

I think it would make strategic sense for SendGrid to greenlight my account. It was not a sensible, necessary anti-spam policy IMO, it was just poor execution.