SMTP (Simple Mail Transfer Protocol) is a reliable and widely used protocol for sending email messages. SMTP provides a standard method for servers to send and receive messages between each other, and it is supported by all major email clients, servers, and web applications.

SMTP has been around for over three decades and has established itself as the standard protocol for sending email messages. With proper configuration and setup, SMTP can reliably deliver emails to the recipients without fail. However, like any technology, there can be issues with SMTP such as server downtime, email delays, or issues with email authentication that can affect email delivery.

To ensure the reliability of email delivery using SMTP, it's essential to maintain proper configuration and management of the email servers, DNS records, and email authentication protocols. Additionally, it's crucial to keep in mind that email security is also a factor that can affect the reliability of the SMTP protocol, and it's important to implement security measures such as encryption and authentication protocols to ensure email integrity and delivery.