The choice on whether to use SMTP or API is completely different and dependent on your needs as well as the capability of the platform you are using to send this emails.

Easy to connect and can connect to any systemFlexibility in sendings
Doesn’t involve any development from your sideGreater security
Better on a larger scale

A cloud based SMTP service might be as effective as a HTTP API and yet an API could offer a bit more flexibility in the level of customizations as well as faster times and relatively low chance of failure due to verification processes.