How to Use SMTP?

The Simple Mail Transfer Protocol (SMTP) is a standard protocol used to send, receive, and relay email messages between servers, mail transfer agents, and mail user agents. Understanding how SMTP works and how to properly configure it is crucial for managing email delivery.

How to Use SMTP?

SMTP utilizes a client-server architecture to route messages through the internet to the correct mail servers and recipients. Here is an overview of using SMTP and best practices for configuration:

Setting up SMTP

To send emails using SMTP, you need:

  • An SMTP server to send outbound messages
  • SMTP credentials like username and password to authenticate
  • Clients and web servers configured to connect to your SMTP server

Many email providers like Gmail or services like Mailgun provide SMTP servers and credentials for sending email:


  • SMTP server:
  • SMTP port: 465 or 587
  • Encryption: SSL/TLS


  • SMTP server:
  • SMTP port: 587
  • Encryption: TLS

Your web server or email client needs to be configured to use your SMTP server and credentials in order to send mail.

Configuring Clients and Servers

Email Clients

In your desktop or mobile email app, add the SMTP configuration settings provided by your email provider or service:

  • SMTP server address
  • Connection security type (SSL/TLS)
  • Port number
  • Login credentials

Web Servers

In languages like PHP, Python, Node.js, Java config files, specify the following:

  • host – SMTP server URL
  • port – Port number
  • security – SSL/TLS
  • username
  • password

This allows contacting the SMTP server to deliver emails.

SMTP Authentication

Most SMTP servers require authentication via a username and password before allowing sending emails, for security and spam filtering.

Common authentication types include:

  • LOGIN – Sends username/password plain text
  • PLAIN – Encodes username/password
  • CRAM-MD5 – Hashed username/password

Check which authentication types your SMTP server allows.

Sending Email

To send an email via SMTP:

  1. Open a connection to the SMTP server
  2. SMTP handshake and connection establishment
  3. Authenticate with username and password
  4. Send email contents including recipients
  5. Close the SMTP connection

The client handles creating the email contents and connecting to SMTP server.

Troubleshooting SMTP Issues

If having issues sending email, check for:

  • Connection/authentication failures
    • Verify SMTP settings are correct
    • Check credentials are valid
    • Retry different ports or security settings
  • Incorrect email addresses
    • Validate recipient addresses
  • Blocked by spam filters
    • Enable authentication in SMTP
    • Check mail content for issues
    • Contact hosting provider

Using command line SMTP clients can help diagnose connection or authentication problems.

Best Practices

Follow these guidelines for stable SMTP operations:

  • Keep software up to date – Use latest security patches
  • Enable SMTP authentication – Reduce spam likelihood
  • Check logs for errors – Diagnose failure root causes
  • Use dedicated IP – Maintain deliverability reputation
  • Monitor blacklists – Check if IP blocked for spam
  • Send test emails – Validate configurations

Careful SMTP setup and monitoring helps ensure reliable email delivery.

Key Takeaways

  • SMTP is used for sending/receiving email between mail servers
  • Configure SMTP settings in email clients and web servers
  • Authentication protects against spam and abuse
  • Validate connections using command line SMTP clients
  • Enable security features and monitor reputation to allow deliverability

Using best practice SMTP setup and maintenance lets you effectively manage mail sending.


Configuring SMTP properly is essential yet simple for delivering emails reliably. By pointing your email client or web server to use a reputable SMTP service with correct settings, authentication enabled, and security features active, you can focus on creating email content rather than worrying about maintain mail infrastructure. Checking server logs, monitoring spam blacklists, sending test messages, and staying up to date give additional confidence your messages will arrive without issues. With a fundamental understanding of how SMTP transfers your messages through the intricate internet network, you can keep your communications flowing smoothly.

Frequently Asked Questions

  1. What is SMTP?
    SMTP stands for Simple Mail Transfer Protocol. It is the standard protocol used for routing and transferring email messages between mail servers over the internet.
  2. How does SMTP work?
    SMTP uses a client-server architecture. The SMTP client opens a connection, authenticates, sends the outgoing email content, then closes the session. The SMTP servers route the message to the correct receiving server closest to the recipient.
  3. What is the difference between SMTP, POP3, and IMAP?
    SMTP transports mail between mail servers. POP3 and IMAP handle retrieving messages from the mailbox for users. POP3 downloads then deletes messages from the server while IMAP allows accessing messages without deleting them.
  4. What SMTP settings do I need?
    The key SMTP settings are server address, port number, connection security protocol such as SSL/TLS, and login credentials for authentication. These settings can typically be obtained from your email provider or SMTP service.
  5. How do I configure Gmail SMTP?
    Gmail’s SMTP server is Use port 465 or 587 for SSL/TLS security and enable authentication via your Gmail username and app password. On the client, provide the Gmail SMTP server, port, SSL/TLS setting, username, and app password.
  6. What’s the difference between port 25, 465, and 587?
    Port 25 is unencrypted SMTP traffic. Port 465 is for SSL encrypted SMTP. Port 587 is typically used for TLS encrypted SMTP.
  7. What are some common SMTP authentication errors?
    Common SMTP authentication issues include incorrect login credentials, restricted IP addresses, disabled authentication methods, blocked ports by the network or firewall, and expired passwords.
  8. Why do my SMTP emails end up marked as spam?
    Potential reasons for emails going to spam include: sending from an unknown IP, lack of authentication, suspicious content like too many links or images, aggressive sending patterns, or domains with poor sending reputation. Enabling SMTP authentication and monitoring such policies can help.
  9. How can I ensure SMTP server security?
    Use encryption like SSL/TLS for traffic, enable SMTP authentication, use strong passwords, keep your server’s software up to date with latest security patches, restrict access at the network level with firewall policies, and monitor logs regularly.
  10. Can I use Gmail or Yahoo SMTP servers to send emails?
    You typically cannot use big public email providers’ SMTP servers, as they restrict use only to their own users. However, they provide users SMTP access to send outgoing mails. For programmatic sending, a third-party SMTP service is recommended instead.
  11. How do I fix SMTP connectivity issues?
    Check that you have an active connection, the SMTP server name is entered correctly, network access to the server is allowed, port number is correct, security settings match protocol, and authenticate successfully. Use command line tools like Telnet to validate the settings and credentials work.
  12. Why SMTP requires connected TCP Sessions?
    SMTP server communicates back and forth with clients over a TCP session. This persistent open connection enables validating recipients, transmitting message content, sending errors for invalid recipients, ensuring secure authentication, and allows reliable disconnection upon completion.
  13. Should I always use SSL/TLS for SMTP?
    It is strongly recommended to always require encrypted SSL/TLS connections for SMTP traffic whenever possible. This protects usernames, passwords, emails contents from being intercepted over insecure networks.
  14. What’s involved in setting up my own SMTP server?
    Setting up a custom SMTP server requires purchasing a server, registering a domain name, installing and configuring SMTP server software like Postfix, integrating with authentication systems, configuring DNS records, opening communication ports, and carefully managing security and email policies.
  15. What are some alternatives to managing my own SMTP server?
    Popular alternatives to handling your own SMTP infrastructure include using a dedicated email provider like G Suite, using a cloud SMTP service like SendGrid, Mailgun, or Amazon SES to only handle outbound delivery, or integrating a cloud function platform with SMTP capabilities like Mailjet.
  16. How can I test my SMTP configurations?
    Great ways to validate your SMTP setup are using Telnet on command line to manually send test messages, using SMTP client libraries in languages like Python to script sending tests mails, or using free online tools like Mail-Tester which runs diagnostics against your domain. This helps catch configuration issues.

Leave a Comment