SMTP vs API Email Sending

Quick contrast

SMTP: libraries in every language; great for transactional SMTP service; you manage MIME.
ESP API: SendGrid/Mailgun HTTP; templates and stats in their UI.
Gmail API: Brahmastra G-Api; Google OAuth; Gmail quotas.

Brahmastra Manual Sender is neither SMTP nor Gmail API — it is a bulk email sender in the Gmail web app. Product page: email sending API.