Gmail does not work, you are right. We currently use SendGrid for our email. It was a little faster to set up than SendInBlue.
The limits on both are such that you will want to ask people not to subscribe to topic updates by email if you have any kind of voulme at all. We blew through 300 emails by like 10am with just a few people subscribed.
I think that's configurable in Node 2.0. Are you guys running that yet? I'd have to go dig back in the guts and see, but I think that you can turn off the ability for people to subscribe to e-mail updates. Realistically they can get pretty granular updates with the built-in watch system without having to get them in their e-mail, since that can be problematic apparently.
Edit: Correction, apparently you can only disable digest e-mails. The issue has come up several times on the forum on how to globally disable the option, and the co-founder keeps marking it as resolved by referring back to a circular chain of asks for it.
Seems weird but ok.