What is SMTP (Simple Mail Transfer Protocol)?

What is SMTP?

SMTP (Simple Mail Transfer Protocol) is the protocol used to send email and relay it between mail servers. When a user sends a message, the email client submits it to an SMTP server; servers then use DNS MX records to find the recipient domain’s mail server.

SMTP is not used to read email. IMAP or POP3 usually synchronizes messages in an inbox. SMTP’s job is to move a message toward the next correct mail server.

How Does It Work?

• Submission: An application or client sends the message to an authorized SMTP server.
• Authentication: A username and password, API key, or service identity is verified.
• Routing: The MX record for the recipient domain is found.
• Relay: The message is transferred to the target mail server.
• TLS: STARTTLS can encrypt the connection.
• Bounce: A failure response is generated when delivery is not possible.

Business Use

SMTP is the basic channel for transactional email such as password resets, order confirmations, invoice notices, support tickets, and system alerts. An application can run its own SMTP server or use a managed email provider.

Deliverability does not end with the SMTP connection. SPF, DKIM, DMARC, IP reputation, rate limits, bounce handling, and template quality should be monitored together. IMAP is different: it is used to read and synchronize messages from a mailbox.