Paying to send emails? I assume you mean something like Twilio? For a company providing this type of service, I would imagine they would have written the core

Paying to send emails? I assume you mean something like Twilio? For a company providing this type of service, I would imagine they would have written the core code themselves and email is pretty basic/simple to send (stuff like texting is much more complicated). I would be shocked if they actually just plugged in something like Twilio to farm out their core service.