I realize you said it depends, but did they lesson the limit of 500 outgoing per account per day?
That's pretty low for any service, and a total hassle to create a bunch of fake user accounts and keep track of how much they've sent today while round robin-ing around "as many accounts as I hope I'll need today".
I think that's the for the free version. There's now a paid version. It's not cheap depending on how many email accounts you need for your startup ($50/year per email account), but it will give you a reliable email provider that gets you through a lot of spam bins
It's good to note to that you can do aliases for each account too
That's pretty low for any service, and a total hassle to create a bunch of fake user accounts and keep track of how much they've sent today while round robin-ing around "as many accounts as I hope I'll need today".