Why SMTP for email?

Sending emails reliably is critical for communication. Simple Mail Transfer Protocol (SMTP) is an essential standard that enables this.

Why SMTP for email?

Simple Mail Transfer Protocol (SMTP)

SMTP is a set of communication guidelines that allows software to transmit email reliably and efficiently. When you send an email from an email client or webmail, SMTP handles sending the message between servers across the internet.

Here is a high-level overview of how SMTP powers email delivery:

  • Your email client or webmail contacts the SMTP server of your email provider over port 25
  • Your SMTP server accepts the message content and recipients
  • The SMTP server adds routing information to the email message
  • Your SMTP server routes the email to the destination SMTP server
  • The receiving SMTP server delivers the email to the recipient’s inbox

Why is SMTP Popular?

SMTP has remained the standard protocol for sending emails since its inception in 1982 due to key advantages:

  • Simplicity – Text-based protocol with easy-to-understand transmission rules
  • Interoperability – Servers and email clients from different vendors can communicate
  • Flexibility – Permits emails with rich content including attachments
  • Reliability – Delivery and send receipts with error checking

Other mail transfer methods have cropped up but SMTP continues to dominate as the fundamental email routing system between mail servers.

Key SMTP Components

SMTP relies on various components working together:

  • SMTP Banner – Identifies SMTP server to connecting servers
  • SMTP Commands – Text commands to initiate handshaking and transmit message
  • SMTP Replies – 3 digit codes indicating status like command success or failure
  • Message Body – The email content and attachments being transmitted

How SMTP Works Step-by-Step

When you hit “send” in your email client, a fascinating sequence happens behind the scenes to route your email across the internet. Here are the step-by-step details:

  1. TCP Connection – Email client initiates a Telnet connection over TCP port 25 to the SMTP server
  2. SMTP Banner – Sender’s SMTP server identifies itself, establishing the connection
  3. HELO Command – Sender’s SMTP server introduces itself to begin communication
  4. Mail Command – Sender email address is specified
  5. RCPT Command – Recipient email addresses are specified
  6. DATA Command – Initiates message content transfer
  7. Message Body – Email content and attachments sent
  8. Stop Command – Sender ends the message body
  9. Confirmation – Receiving SMTP server confirms if the message is safely stored
  10. SMTP Conversation ends – Connection closes with a quit command
  11. Routing – Receiving SMTP server identifies recipient mail server and routes email
  12. Delivery – Recipient SMTP server delivers email to recipient inbox

Understanding this SMTP architecture helps troubleshoot email delivery issues like incorrect routing or rejected recipients.

Key SMTP Commands

The core SMTP commands facilitate server-to-server communication:

  • HELO – Initiates the SMTP conversation
  • MAIL FROM – Specifies sender email address
  • RCPT TO – Specifies recipient email address
  • DATA – Initiates message content transfer
  • RSET – Aborts current mail transaction
  • NOOP – Pings server to keep connection alive
  • QUIT – Closes the SMTP connection

There are also various extended SMTP commands enabling capabilities like authentication and encryption.

SMTP vs API Email Services

Many services bypass SMTP and use API endpoints to transmit emails. For example, Mailgun provides a REST API to send messages. Here’s how SMTP and API email services compare:

Feature SMTP API Email Services
Infrastructure Dependency Relies on SMTP servers Email delivery coded into app
Control Less flexibility to customize delivery Fine-tuned programmatic control
Scalability Limited by SMTP infrastructure Auto-scales to traffic spikes
Monitoring Logs provide some visibility More real-time tracking and analytics
Security Typically less secure Extra encryption possible

When to Use SMTP or API Email

Given the pros and cons, here are good use cases for each email transmission method:


  • Sending small volumes of email
  • Basic delivery needs
  • Legacy applications

API Email Services

  • Dynamic content in emails
  • Large volumes of email
  • Precise delivery monitoring required
  • Heightened security important

So SMTP handles typical email loads well. But for more complex needs, API services provide greater flexibility and capability.

Key Takeaways

  • SMTP enables routing between mail servers to deliver emails
  • Understanding SMTP helps troubleshoot email issues
  • API services provide more advanced email functionality

Email is mission-critical. Whether using SMTP or API services, it pays to optimize email delivery and reliability.


SMTP has reliably enabled global email communications for 40+ years. The straightforward protocol continues going strong as the foundation for routing mail between servers. API services are emerging to meet more advanced email delivery needs. Following SMTP best practices and monitoring key metrics helps ensure your emails consistently reach inboxes.

Frequently Asked Questions

  1. What port does SMTP use?
    SMTP typically communicates over port 25 for unencrypted connections. Ports 465 or 587 are also used for secure SMTP connections.
  2. Is Gmail SMTP or API?
    Gmail uses SMTP for sending outgoing emails. But incoming emails are retrieved using Gmail API for greater speed and reliability.
  3. What are common SMTP errors?
    Some frequent SMTP errors include misconfigured DNS, blocked ports, greylisting, invalid recipients, max send limits exceeded, and domain reputation issues like blacklisting.
  4. Does SMTP require authentication?
    Earlier versions of SMTP did not require authentication. But most modern SMTP servers implement authentication mechanisms like SMTP Auth to improve security.
  5. Can SMTP send attachments?
    Yes, MIME encoding enables SMTP to transmit binary attachments like images, audio, documents across ASCII network connections.
  6. What is the difference between SMTP and IMAP?
    SMTP focuses only on sending emails while IMAP handles retrieving messages from the mail server to the email client for users to read.
  7. Should SMTP traffic be encrypted?
    Encrypting SMTP connections with SSL/TLS is considered a email security best practice to protect usernames, passwords, emails contents from surveillance.
  8. Is SMTP still used today?
    Despite new technologies, SMTP remains the most widely-used standard for routing emails across the internet between mail servers.
  9. What tools can monitor SMTP traffic?
    SMTP monitoring tools like SolarWinds, ManageEngine, and Paessler PRTG can provide visibility into server traffic metrics, connections, errors.
  10. Does Office 365 use SMTP?
    Yes, Office 365 relies on SMTP for routing outbound messages from its mail servers to external recipients over the public internet.
  11. What is the difference between internal and external SMTP?
    Internal SMTP servers route mail within an organization’s network, while external SMTP servers interface with destination domains on the public internet.
  12. Can I use Gmail SMTP to send emails from my domain?
    Yes, most email providers like Gmail enable configuring credentials on non-Gmail clients/servers to relay mail through their SMTP servers.
  13. What is the latest version of SMTP?
    SMTP is still an evolving protocol, with recent extensions adding capabilities like internationalization support, improved authentication, and queue management.
  14. Is Sendgrid SMTP or API?
    Twilio Sendgrid provides both SMTP servers and Web API endpoints to choose from for programmatically sending high volumes of email.
  15. What tools can test an SMTP server?
    Useful SMTP testing tools include Telnet, swaks, Mail-Tester, smtptest, Mailtrap, and MailSlurper which validate connections, authentication, deliverability.
  16. Can I limit usage of my SMTP server?
    Set restrictions in your SMTP config for connection limits, messages sent per hour,max message size allowance, approved sender lists to constrain resource usage.
  17. What language is SMTP written in?
    The core SMTP spec is written in ASCII text. But SMTP servers and clients interacting via the protocol are coded in various languages like C, Java, C#, Python.

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