What is SMTP?
SMTP (Simple Mail Transfer Protocol) is the standard internet protocol used for sending and relaying email messages between mail servers, operating on port 25 for server-to-server communication and ports 587 or 465 for client-to-server submission.
How Does SMTP Work?
SMTP handles the outbound side of email delivery. When you send an email, your email client connects to an SMTP server and transmits the message, which is then relayed through one or more intermediate SMTP servers until it reaches the recipient's mail server. The protocol uses a series of text-based commands and responses to negotiate the transfer, including identifying the sender, specifying recipients, and transmitting the message body.
In web hosting, proper SMTP configuration is critical for reliable email delivery. Many hosting providers include an SMTP server with their plans, but websites that send high volumes of email (such as newsletters or transactional messages) often use dedicated SMTP relay services like SendGrid or Amazon SES to improve deliverability and avoid blacklisting. Modern SMTP implementations require authentication (SMTP AUTH) and encryption via STARTTLS or SSL/TLS to prevent unauthorized use and protect message content in transit.