We have an in-house (Golang based) rendering service that, besides adding bunch of config values to email, simply merges the template with data. The next step is to send these rendered emails via SendGrid.
I can see from SendGrid's API documentation that they offer REST api and SMTP api. Lets say, if we want to send out 600k emails, then making 600k rest requests, even with some concurrency, wont be the most efficient way. I also went through their article about choosing SMTP or REST api but no clear answer there as well.
What is most preferred/efficient route for the situation described above?
I use to send to sendgrid through basic smtp. I'd get about 3 emails a second going out that way. I switched to their api where I basically send about 500 per json POST and i find each of those posts takes a couple 100 ms max so now, I can send 15,000 emails in minutes max. Sendgrid does get that out fast also which is quite nice.