The pipe

Client → SMTP AUTH → relay → MX of the recipient domain. Failures are 4xx/5xx. Transactional SMTP service and email SMTP service are the same protocol with different pricing and abuse rules.

How to send bulk emails using SMTP

Get credentials from SES, Mailgun, or SendGrid, set host/port, send MIME. Brahmastra is not that client. We send through Gmail UI or Gmail API. See bulk SMTP vs Brahmastra.

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