What is SMTP full form?

The Simple Mail Transfer Protocol (SMTP) is a communication protocol for electronic mail transmission. SMTP handles sending messages from one mail server to another mail server or mail transfer agent. It defines the message format and the protocol for exchanging messages between servers.

What is SMTP full form?

How SMTP Works

When you send an email from your email client, here is what happens behind the scenes:

  • Your email client connects to your local SMTP server using port 25
  • Your local SMTP server verifies your identity
  • The email content is transmitted to the local SMTP server which adds routing information to the top and packages the email for delivery
  • The local SMTP server looks up the domain of the recipient and finds the SMTP server responsible for accepting messages
  • It then hands off the email to that SMTP server which delivers it to the recipient’s email inbox

So in summary, SMTP is responsible for routing and delivering your email messages to the recipient’s email server.

Core Components

These are the core components that enable SMTP email delivery:

  • Mail User Agents (MUA) – The email client that you use for drafting and sending your emails e.g. Outlook, Thunderbird etc.
  • Mail Submission Agents (MSA) – Receives the email from your MUA and establishes connection with SMTP server.
  • Mail Transfer Agents (MTA) – The SMTP servers responsible for routing and relaying emails to recipients using SMTP protocol and DNS MX records.
  • Mail Delivery Agents (MDA) – Receives the email from the last MTA and saves it to the recipient’s mailbox where it can be accessed by email clients.

Main Functions and Features

SMTP was designed to provide the following key functionality for transmission of email:

Routing Email Using DNS MX Records

SMTP uses Domain Name System (DNS) MX records to determine the SMTP mail server responsible for accepting messages addressed to a specific domain. This allows messages to be automatically routed to the correct destination mail servers.

Verifying Identities Using SMTP Authentication

SMTP servers can require clients to authenticate using mechanisms like OAuth2 and NTLM before allowing sending of emails, ensuring only authorized users can relay mails through the server.

Encryption Using STARTTLS

STARTTLS extension to SMTP allows the client and server to upgrade a plain text connection to an encrypted Transport Layer Security (TLS) connection to secure email transmission from snooping or tampering during transit.

Store and Forward Mechanism

SMTP employs a store and forward mechanism where an email is passed from one SMTP server to the next until reaching the final destination. This ensures guaranteed delivery.

SMTP Ports and Protocols

Port 25

The default TCP port used for SMTP traffic is port 25. This port must be open on the server for sending and receiving SMTP mail.

Port 465

SMTP over SSL uses port 465 by default for secure encrypted connections. Both the client and server connect via SSL immediately rather than upgrading plaintext communication to TLS.

Port 587

The default SMTP port for “message submission” i.e. allowing clients to send emails to the mail server is port 587 using STARTTLS encryption.

Common SMTP Server Platforms

Some commonly used SMTP servers and platforms are:

  • Sendmail
  • Postfix
  • Microsoft Exchange
  • Oracle Communications Messaging Server
  • MailMarshal SMTP
  • MailEnable Standard

These feature-rich platforms provide enterprise-grade mail delivery with security protections, analytics, integrations with mail clients and more. Self-hosted open source SMTP like Sendmail is also popular.

Self-hosted vs Cloud-based

Instead of managing your own servers, you can also use fully managed cloud-based SMTP services like SendGrid, Mailgun, SparkPost and Amazon SES without having to worry about security, deliverability and maintenance.

SMTP Security Best Practices

Here are some recommendations to ensure security of your SMTP infrastructure:

  • Require SMTP authentication of all users to prevent unauthorized mail sending
  • Enable TLS encryption using STARTTLS between hops
  • Use DKIM and SPF to prevent email spoofing and improve deliverability
  • Monitor your SMTP traffic and logs for signs of compromise or malicious activity
  • Keep your SMTP gateway updated and patch security vulnerabilities

Additionally, proper mail server hardening, isolating internal mail servers from the internet, implementing firewall policies and monitoring reputation are also advised.

Protecting Against Email Attacks

These days cybercriminals are constantly coming up with innovative email attack techniques like:

  • Phishing attacks to steal login credentials
  • Malware distribution through malicious attachments
  • Spam campaigns spreading disinformation
  • Business Email Compromise (BEC) scams impersonating trusted contacts

That’s why a robust email security strategy with solutions like anti-spam filters, antivirus, sandboxing, URL reputation and user education is a must.

Key Takeaways

  • SMTP or Simple Mail Transfer Protocol is a standard for transmission of email messages between servers over the internet
  • Clients use port 25 and 587 to send emails which are relayed by SMTP servers using DNS MX records till they reach recipient
  • Security mechanisms like SSL/TLS encryption, SMTP auth and others should be utilized to protect email delivery
  • Leading SMTP platforms provide reliable and scalable email solutions with security and analytics
  • Guarding your infrastructure and users against emerging email threats is equally important


In closing, SMTP powers and enables the global email ecosystem allowing mail servers to communicate with one another for routing billions of messages across the world each day. Mastering both the technology and security best practices of running SMTP infrastructure is critical for any organization to productively collaborate and communicate through email.

Frequently Asked Questions (FAQ)

What does SMTP stand for?

SMTP stands for Simple Mail Transfer Protocol. It is an internet standard protocol for sending email messages between mail servers and routing them to the correct recipients.

Who invented SMTP?

SMTP traces its origins to two RFC standards in 1982 – RFC 821 and RFC 822 which defined Internet email architecture comprising of SMTP for transmission and message format respectively.

What is the difference between SMTP and POP3?

SMTP is for sending emails while POP3 is for receiving emails. SMTP allows sending outbound mails to others while POP3 enables users to retrieve their received emails from the server.

What port does SMTP use?

The default TCP port used by SMTP is port 25. Other common ports are 465 for implicit SSL/TLS traffic, and 587 for submission using STARTTLS encryption.

How does SMTP work with DNS?

SMTP uses Domain Name System (DNS) MX resource records to lookup and find destination mail servers across the internet responsible for accepting email traffic for any domains specified in the SMTP envelope and routing address.

How are emails sent using SMTP?

An SMTP client initiates a connection to the SMTP server, authenticates itself, constructs message content/attachments and senders/recipients data, asks SMTP server to deliver email by specifying SMTP commands and associated data and finally closes the connection when transmission ends.

How do I choose an SMTP provider?

When selecting an SMTP service, look for ease of use APIs, deliverability to major ISPs, protection against email threats like spam/phishing/viruses, email analytics, reputation management, integrations, reliability and support channels for assistance.

What is SMTP authentication?

SMTP authentication is the mechanism of confirming a user’s identity using technologies like multi-factor authentication or client certificates before allowing to relay mail through the SMTP server to other domains which prevents unauthorized use.

How do I secure my SMTP server?

Some ways to secure an SMTP server are require authentication before send, enable TLS encryption between hops, implement firewall policies, monitor traffic/logs for anomalies, patch regularly, harden configurations and disable risky functionality. Solutions like DMARC and DKIM also improve email security.

Do email clients use SMTP?

Yes, email clients include SMTP functionality to connect to your email provider’s outbound SMTP server in order to securely compose, send and transmit your messages to recipients across domains and servers over the internet leveraging SMTP.

What are SMTP error codes?

SMTP servers can return various standardized response codes starting from 500-599 indicating an error like 501- Syntax error in parameters/commands, 550 – Mailbox unavailable, 554 – Transaction failed etc which help troubleshoot issues.

What tools can I use to test SMTP?

Some useful tools for testing SMTP connectivity and delivery are Telnet, PuTTY, Open SMTP Catcher, Mail Tester, Mail-Tester, smtp4dev, swaks and smtp-source. They allow manually interacting and simulating sending of test emails through any target mail server via SMTP protocol.

How do I troubleshoot SMTP connectivity issues?

Check for firewall or ports blocking TCP 25/587 traffic; Use Telnet or OpenSSL s_client to manually check server response; Verify correct DNS records allow routing mail; Inspect SMTP logs closely for errors; Test with authentication disabled/enabled; Check if IP is on any blocklists.

What security checks should be done on an SMTP server?

Scan for open relays and vulnerabilities; Verify TLS encryption enforced for sessions; Check auth mechanisms implemented and auth required for all; Inspect configurations & disable unnecessary functions; Monitor deliverability and flow logs; Implement DMARC/SPF/DKIM for security.

Which companies provide SMTP services?

Some leading SMTP service providers are SendGrid, Mailgun, SparkPost, Mailjet, Amazon SES, Microsoft 365, SMTP2GO, Pepipost, MailSlurp and SendInBlue among others. They offer developer friendly APIs, security and analytics.

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