I am using SocketLab Email services, which allow bulk email sending. Their service is cheaper than Sengrid and they were recently featured in Appsumo, which means they’re likely to gain a lot of traction.
There is a simple injection REST API that lets users send e-mails from their apps (doc here: https://inject.docs.socketlabs.com/v1/email/json-request).
I tried to create a plugin myself, but I am not that familiar with APIs. Wondering if someone could create that plugin.