Yes, it’s tricky. We likely cannot send emails in free apps because it would be abused right away, and our domain would get shut down. So, it will likely need to be a Pro feature, where you would need to have your own SendGrid account or somehow configure your own domain.
I’d love to let free apps do 10-100 emails per month for free, but we can’t because it will be abused 
Totally agree with you. It’s a pro feature for sure!
No experience of Zapier but with Integromat you can simply provide your own SMTP details and it’ll use those for sending email. That gives the app developer lots of control and the burden of capacity and price is on them. Additionally, when used in conjunction with a “new row in sheet” trigger, Integromat uses something very similar to a Glide template to build the content using a combination of free text and column values. I imagine that we’re going to need integrations like this if / when we go to Glide native sheets.