SMTP is more reliable than default Magento mail because it uses authenticated servers to send emails securely. A Magento 2 SMTP Extension ensures proper email routing through trusted SMTP providers, reducing failures and delivery issues. Unlike basic PHP mail functions, SMTP supports encryption methods like SSL and TLS, which improve security and sender reputation. It also minimizes the chances of emails going to spam, resulting in higher deliverability, better performance, and more consistent communication with customers.