Why is SMTP needed?

Simple Mail Transfer Protocol (SMTP) plays a vital role in the transmission of email over the internet. Here’s why it’s a crucial part of email infrastructure:

Reliable Delivery

SMTP is designed to provide reliable delivery of email messages. It uses TCP to create a persistent connection and ensure data packets are transmitted error-free between email servers. This prevents loss of messages over unreliable networks.

Why is SMTP needed?


As an open standard protocol, SMTP enables interoperability between email services and servers, regardless of the underlying operating systems or programming languages used. Any system can connect to the SMTP port of another system and transmit messages successfully.

Established Framework

Released in 1982, SMTP provides a well-established framework for email transmission with clearly defined commands and procedures. This ensures consistency in how different systems and networks transmit email.


SMTP only deals with transmitting the envelope and content of messages. Additional functionality like message storage and mailbox management are handled separately. This separation of concerns provides flexibility to developers.


SMTP’s simplicity enables it to handle huge volumes of emails without extensive overhead. Large-scale email providers are able to serve billions of accounts reliably using SMTP as the base protocol.


Mechanisms like SMTP TLS provide transport layer security by encrypting communication between email servers. SMTP strictly follows identification and authentication procedures as well.

Here are some key reasons why SMTP is an essential part of email infrastructure across the internet:

  • Store-and-Forward – Messages are relayed and stored temporarily on each server along the transmission path till they reach the recipient’s server. This makes email delivery robust.
  • Push Protocol – Unlike POP3 which requires clients to poll and pull messages, SMTP pushes messages out to recipients without needing user intervention.
  • Error Handling – Accurate error reporting allows senders to troubleshoot issues like invalid recipients which could cause message delays. Retry schedules can also resume failed transmissions.
  • Simplifies Architecture – Isolating mail transfer to SMTP servers keeps client-side applications simple with no need to handle routing, delivery issues etc.
  • Traffic Management – Throttling, spam filters and other controls at SMTP gateways regulates email traffic and enhances quality of service.

So in summary, SMTP may have been conceived decades ago but remains indispensable for global email services at scale to operate smoothly. Replacement protocols haven’t achieved the same ubiquity and reliability. For the foreseeable future, SMTP will continue serving as the routing mechanism powering modern electronic communication.

How does SMTP work?

SMTP facilitates email delivery through a client-server architecture:

1. Establish Connection

The SMTP client opens a TCP connection on port 25 or 587 to initiate a session with the target mail server. This allows two-way communication.

2. Identify Sender

The client sends the sender’s email address in the MAIL FROM command. This verifies the message origin for tracing and spam checks.

3. Identify Recipient

One or more RCPT TO commands provide the email addresses being sent the message. The server checks if the accounts exist.

4. Send Message

The client transmits the actual message after the DATA command. This includes email headers like subject, date as well as the body content.

5. Close Connection

Once the message transfer completes, the QUIT command terminates the SMTP session and closes the TCP connection between client and server.

Behind the scenes, additional SMTP servers may relay messages till they reach the recipient’s mail server which deposits it in the appropriate mailbox where it can be accessed via POP or IMAP.

Key Commands

SMTP uses text-based commands issued by the client, validated and responded to by the email server:

Command Description
HELO Initiates connection and introduces client to the server
EHLO Extended HELO, indicating extended SMTP features supported
STARTTLS Enables encryption for communicating securely
AUTH Sends login credentials for client authentication
MAIL FROM Sets the message sender’s email address
RCPT TO Provides receiver’s email address
DATA Initiates message content transfer
QUIT Closes the SMTP session after message sent


SMTP differs greatly from HTTP, the protocol used to access web pages:


  • Optimized for email routing
  • Operates on TCP port 25 or 587
  • Text based commands
  • Persistent connections
  • Push-based delivery


  • Optimized for web content
  • Operates on TCP port 80
  • Follows request-response model
  • Short-lived connections
  • Pull-based delivery

So while your browser uses HTTP to retrieve webpages hosted on remote servers, your email relies on SMTP for reliable delivery across networks.

How is an SMTP server beneficial?

Operating an SMTP server in your environment has numerous benefits when it comes to sending and receiving emails.

Enhanced Reliability

An SMTP server located on-premises eliminates dependency on any single external email provider. This improves uptime and reliability for critical communication.

Increased Performance

On-premises servers on a LAN provide higher email throughput and lower latency compared to cloud services affected by internet bottlenecks.

Greater Control

Managing your own SMTP server allows customization of email routing and security policies as per your needs instead of being restricted by vendor limitations.

Cost Savings

Avoiding usage quotas and limiting outbound transmissions can substantially reduce operating costs compared to paid cloud email services.

Auditing & Monitoring

Locally storing email data allows simpler auditing for compliance purposes. Server logs can also be inspected to monitor performance.

Enhanced Security

Vetting external connections through a local SMTP gateway allows blocking suspicious traffic. Emails also remain within your controlled network.

Improved Analytics

Tracking engagement metrics for different campaigns helps measure ROI. Many SMTP servers provide built-in analysis tools.

For these reasons, organizations often utilize SMTP servers behind the firewall that serve as smart relays for their public facing mail infrastructure. This best-of-breed approach improves deliverability through features like caching and reputation filters available from specialised SMTP services.

Key Considerations

When assessing SMTP server solutions for your environment, factor in:

Protocol Support – Make sure the server handles core protocols like SMTPS, SMTP with TLS, POP3, and IMAP for flexibility.

Inbound Filtering – Check anti-spam and anti-malware scanning capabilities along with greylisting options.

User Authentication – Support external sources via LDAP or CASB for single sign-on.

Storage Options – On-server mailbox databases, network attached storage, cloud hosted?

Performance & Scalability – Evaluate delivery throughput and compute resources to handle peaks.

Centralized Management – Can you manage users, policies, access etc through one admin console?

Monitoring & Reports – Inspect logs and email statistics to meet governance needs.

High Availability – Consider clustering, load balancing and failover to eliminate single point of failure.

Choosing the right SMTP platform lets you optimize email delivery as per your specific requirements while addressing modern threats.

Key Benefits of SMTP

Despite newer protocols, SMTP remains the standard for email transmission due to longstanding advantages:


SMTP’s status as the de-facto email routing protocol guarantees interoperability with third-party servers and services.


The core SMTP spec and commands have changed little over 40 years assuring backwards compatibility between clients and servers.


SMTP’s efficiency at bulk message transfer has allowed large providers to scale worldwide delivery operations.


SMTP extensions via ESMTP add new functionality like SMTP over TLS without compromising base protocol.


Spooling, error checking, retry mechanisms and store-and-forward model provides highly reliable email delivery.


SMTP only handles transport allowing choice of mailbox access protocols like POP3 or IMAP for end-users.


SMTP TLS, SMTPS, authentication mechanisms and reputational blocking bolster security against spoofing, abuse and spam.

Key Takeaways

  • SMTP enables reliable routing of email messages between servers over port 25/587
  • HELO, MAIL FROM, RCPT TO, DATA and QUIT are key commands
  • Offers interoperability between email services using open standards
  • Store-and-forward model and push mechanism ensure delivery
  • On-premises SMTP servers provide control, security and delivery optimization
  • Ubiquity, scalability and stability make SMTP indispensable for modern email


Created in an era when networked computing was still nascent, SMTP fundamentally enabled personal and commercial electronic communication to flourish worldwide. The resilience of its core design continues to handle modern mail volumes unimaginable during its inception. SMTP solved the problem of reliable message transfer in a generalized manner so email services could focus on end-user features. While proprietary alternatives and extensions exist, SMTP remains the common language binding global inboxes. New transport protocols may emerge, but SMTP’s dominance for mission-critical mail routing is uncontested given decades of proven operation. Going forward, SMTP will continue to quietly facilitate trillion-plus emails delivered annually.

Frequently Asked Questions

Q: What port does SMTP use?
A: SMTP traditionally uses TCP port 25 for unencrypted connections. Port 587 is used for encrypted SMTP over TLS connections by default.

Q: Is SMTP faster than HTTP?
A: Yes, SMTP is optimized for email transfer and hence offers higher throughput on average compared to HTTP web traffic.

Q: What is the difference between SMTP and SMTPS?
A: SMTP refers to plain text unencrypted sessions while SMTPS indicates SMTP over Transport Layer Security (TLS) for secure connections.

Q: Does SMTP require 3 way handshake?
A: Yes, like any TCP-based protocol, a 3-way handshake opens an SMTP connection which stays established for the entire mail transmission session.

Q: Can I use Gmail SMTP server?
A: Yes, Gmail offers users an SMTP relay for sending outgoing mail through smtp.gmail.com on port 587 with authentication.

Q: How do I choose an SMTP service provider?
A: Factors like deliverability, throughput, security controls, reputation monitoring, and uptime guarantees matter when choosing an SMTP delivery service suited for your specific use case.

Q: What tools can test SMTP servers?
A: Tools like swaks, Mail Tester, smtpping, and others help diagnose connection, authentication issues or validate mail delivery for a given SMTP server.

Q: Can I route outgoing mail to an external SMTP provider?
A: Yes, MX records and mail forwarders can relay outbound messages from your domain’s mail server to a third-party SMTP service with higher deliverability.

Q: Does SMTP use symmetric key encryption?
A: No, popular SMTP encryption protocols like STARTTLS instead use asymmetric public-key techniques like Diffie-Hellman for secure key exchange.

Q: What is SMTP used for other than email?
A: While focused on email operations, SMTP’s file transfer abilities can be used for general automated data exchange between systems over TCP/IP networks.

Q: Can SMTP integrate with databases?
A: Yes, SMTP servers often integrate with SQL databases and NoSQL data stores to enable query results and monitoring information to be reported over email.

Q: What elements comprise an email address?
A: The local-part before @ and the domain name after @. The domain must have valid DNS MX records that resolve to the mail server addresses.

Q: Is ESMTP an alternative to SMTP?
A: No, ESMTP is an extension that adds additional functionality while retaining SMTP’s core capabilities. All ESMTP systems/clients are inherently SMTP compatible.

Q: Does Outlook use SMTP?
A: Yes, behind the scenes Outlook utilizes SMTP for sending outgoing messages and other protocols like IMAP/POP3 for receiving mail depending on configuration.

Q: What is the most popular SMTP server software used today?
A: Postfix and Microsoft Exchange Server are the most common SMTP implementations within enterprises currently. Sendmail historically led SMTP server market share.

Q: What is the difference between message transfer agent (MTA) and mail server?
A: MTAs specialize in routing SMTP mail traffic between domains while mail servers provide additional email access protocols like POP3, IMAP for end-users.

Q: What are common troubleshooting steps for SMTP issues?
A: Checking DNS records, validating SMTP connectivity via Telnet session, inspecting server logs/rejection responses, and testing authentication workflows help troubleshoot.


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