> Postmark - If you’ve launched your product, are charging for it, and haven’t taken outside investment, contact the Postmark support team and they'll give you $75 account credit to help with your email costs.
A lot of these discounts are based on the assumption that you're a VC-backed, or about-to-be-VC-backed startup, that will have enough runway and growth to stick around for a couple years, and will smoothly transition from a steep discount to a 4-digit monthly bill around the time you land your Series A or whatever. I like Postmark's because it's much more reasonable - "you _don't_ have VC money, but you have an obvious path to making money (and thus being able to pay us), so we'll give you some credit for free as you get up and running."
I've been totally freeloading for a side project of mine that I _don't_ expect to make any money, but in researching cheap plans I came across a lot of these sorts of discounts (usually while looking to see if they had an open source discount, since my side project is open source). It's an interesting gamble, but I respect Postmark's the most, I think.