SMTP (Simple Mail Transfer Protocol) is a network protocol used for sending email messages between servers. It defines how emails are transmitted from a sender to a mail server and between mail servers across the internet.
SMTP is the standard protocol responsible for outgoing email delivery in most email systems.
Table of Contents
How SMTP works
SMTP operates using a client–server model.
When an email is sent:
- the email client connects to an outgoing mail server
- the server processes the message and identifies the recipient domain
- the message is forwarded to the recipient’s mail server
- the recipient server stores the message for later retrieval
SMTP is only responsible for sending and relaying email. Receiving email is handled by other protocols such as IMAP or POP3.
SMTP communication process
An SMTP session follows a structured sequence of commands between systems:
- connection is established over TCP
- the sending server identifies itself
- sender and recipient addresses are specified
- message content is transmitted
- the session is closed after delivery
This process ensures standardized communication between different mail systems.
Ports used by SMTP
SMTP typically uses the following ports:
- Port 25 – server-to-server email transfer
- Port 587 – email submission from clients (recommended with authentication)
- Port 465 – secure SMTP using SSL/TLS (legacy but still supported in some systems)
SMTP and email retrieval
SMTP is used only for sending emails.
To receive and read emails, different protocols are used:
- IMAP – synchronizes emails with the server
- POP3 – downloads emails to a local device
Security considerations
SMTP was originally designed without encryption. Modern implementations use additional security layers such as TLS to protect communication.
Without encryption, email content and credentials may be exposed during transmission.
Secure SMTP configurations use authentication and encrypted connections to prevent unauthorized access and interception.
Practical use cases
SMTP is used for:
- sending emails from email clients (webmail or applications)
- relaying emails between mail servers
- sending automated system notifications
- integrating email functionality into applications and websites
Practical implications
SMTP is essential for email delivery but does not store or display messages.
Proper configuration of SMTP servers is required for reliable email delivery and to avoid issues such as spam filtering or rejected messages.
In modern hosting environments such as mybox, SMTP is typically used together with secure authentication and encryption.
Summary
SMTP is the core protocol for sending email messages across the internet. It enables communication between mail systems using a structured relay process and is essential for all outgoing email delivery.