I had a hit on HN with a side-project last year [0]. It delivers public domain books in chapters to your email. So I was needing to send over 1,000 emails a day from day one basically.
I initially used SendGrid for my project, but SendGrid's free trial had a limit of 100 emails a day. Well, I just needed to upgrade, right? Wrong. Sendgrid had a rule where it was impossible to upgrade from the free trial until the trial period was over. So 90% of my emails were not being delivered because I wasn't in a paid plan and I couldn't upgrade to a paid plan.
The worst part is that it wasn't very straightforward to understand I was in that catch 22. It took me about 4 or 5 days of sending their customer support messages and them taking too long to reply and not being very clear in their replies to understand the whole issue. And even proving that my traffic was legit (the HN post was enough evidence that I was not sending spam I think) was not enough for them to allow me to upgrade.
So I immediately changed to MailGun. They had similar worries about a new account with significant email traffic, but I sent an email to customer support right after signing up, they replied in an hour or so already allowing me to upgrade. Never had an issue with MailGun and I am very happy with their tech and service.
I am now creating a business on top of email delivery and staying with them. Hope this acquisition doesn't change much in that front.