We recommend using the best suitable method depending upon your application capabilities and feasibility requirements.
Below given are few facts for your information on HTTP API integration:
1. It is faster than SMTP
2. You can send emails at a much larger scale than SMTP
3. Saves your server resources as the Email generation is taken care of by us.
On the other hand, SMTP is easy to integrate as you just need to replace your existing SMTP credentials along with SMTP relayhost within your application.