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What / Why / How and Requirements for SMTP


Last updated on Sep 14, 2023

Overview of SMTP Protocol

The internet has revolutionized communication around the world. Today, sending an email is as easy as typing a few words and clicking send. But have you ever wondered what happens behind the scenes to get those emails delivered securely across the globe?

This is where SMTP comes in. SMTP or Simple Mail Transfer Protocol forms the backbone of internet email and ensures your messages reach the intended recipients reliably. Let's start with the basics of how this ubiquitous protocol functions.

What is SMTP?

SMTP is a set of communication guidelines that allows software to transmit electronic mail over the internet. It is a relatively simple, text-based protocol that uses TCP ports to establish connections and transfer data.

As an application layer protocol, SMTP handles only the packaging and routing of email messages, not their content. It defines the message format as well as the sequence of commands to initiate mail exchanges between servers. Clients like email services and mail apps rely on SMTP to send outgoing mails to the recipients' servers which then store them for access.

Here's a quick 101 on how the SMTP process works:

- The sending mail client contacts the SMTP server and introduces itself

- It provides information about the sender, recipients, and mail content

- The server routes the mail to the next SMTP server closer to the recipients

- This relaying goes on until the mail reaches the recipients' mail servers

- The recipients then access the emails using protocols like POP3 or IMAP

So in essence, SMTP handles only the sending or “pushing” of the mails across. That's why you need protocols like POP3 or IMAP to “pull” mails on the receiving end and make them available in your inbox.

How does SMTP work?

The SMTP model relies on a direct connection between hosts to transfer mails. Here is a simplified play-by-play:

1. The sending client opens a TCP connection to the SMTP port on the destination server

2. It introduces itself with the `HELO` command along with its domain name

3. The `MAIL FROM:` command indicates the sender's email address

4. The `RCPT TO:` command provides the recipient's email

5. The client sends the message line by line following the `DATA` command

6. The `QUIT` command closes the connection after the transfer is complete

Of course, there are additional nuances like error handling, security extensions etc. But this is the basic flow - connect, introduce sender/recipient, send data, disconnect.

The relaying of mail between multiple servers works similarly. The sending server becomes the client and connects to the next receiving server to forward the mail. This store-and-forward mechanism continues until the mail reaches its destination.

Core components of SMTP

There are primarily four key agents that enable the SMTP mail transfer:

- Mail User Agent (MUA) - The email client like Gmail, Outlook etc. that handles composing and sending mails for the user

- Mail Submission Agent (MSA) - Receives mails from MUAs and hands them to MTAs for relaying

- Mail Transfer Agent (MTA) - Transfers mails between servers using SMTP

- Mail Delivery Agent (MDA) - Receives mails from MTA and delivers them to mailboxes for access

Together, these agents allow submission, relaying, and final delivery of mails through SMTP. The user mostly deals with the MUA while the others run in the background transferring mails between hops reliably.

Advantages and disadvantages of SMTP

Like most protocols, SMTP too has its pros and cons:


- Ubiquitous - Supported by almost all mail servers and clients globally

- Interoperable - Can communicate easily across diverse networks and systems

- Mature protocol - Refined over decades of use making it efficient and reliable

- Text-based - Simple to implement, extend and troubleshoot

- Push-based - Proactive sending mechanism ensures mails are not lost

- Bulk friendly - Built for high-volume mail transfers


- Not secure by default - Vulnerable to exploits without encryption

- Relatively slow - Multiple hops and commands increase latency

- ASCII only - Requires encoding for non-text data

- Delivery delays - Store-and-forward between hops can hold up mails

- No user authentication - Prone to abuse without extension mechanisms

- Pull needed for access - Protocols like POP3/IMAP required for delivery

While SMTP has some shortcomings, global email infrastructure crucially depends on it today. Its capabilities have been enhanced over time with extensions like ESMTP that add security, authentication, and support for multimedia content.

When configured properly with appropriate checkpoints, SMTP offers a robust foundation for worldwide communication via email. Understanding how this ubiquitous protocol ticks is key to using it effectively as an individual and protecting against potential attacks at scale.

History and Evolution of SMTP

Like Rome, SMTP wasn't built in a day either. The protocol has evolved over decades of use on the internet. Let's take a brief tour down memory lane to see how SMTP came into being and transformed over time.

Original SMTP RFC 821 (1982)

It all started in 1982 when SMTP made its debut through the **RFC 821** standard published by Jonathan B. Postel. He laid the foundation by formally defining commands like `MAIL`, `RCPT`, `DATA`, and error codes which are still relevant today.

Some key points about original SMTP:

- Provided basic mail transfer capabilities using TCP

- Defined command/response sequences and message format

- Sender and receiver hosts communicated directly

- No encryption or authentication mechanisms

- ASCII-only message content allowed

So in essence, RFC 821 enabled hosts to transfer mails reliably using SMTP's store-and-forward approach. But it was quite barebones otherwise with lots of room for improvement on security and functionality.

Updated SMTP RFC 5321 (2008)

Over the next few decades, SMTP's capabilities were expanded through various RFCs that extended the original standard:

- RFC 1651 (1994) - Extension mechanisms for commands, enhanced status codes

- RFC 1869 (1995)** - SMTP service extensions like size limits, authentication

- RFC 1870 (1995)** - Message size declaration

- RFC 1985 (1996) - SMTP authentication

- RFC 2554 (1999) - SMTP over TLS for secure transfer

Finally, in 2008, RFC 5321 was published that consolidated all these extensions and updates while obsoleting the original RFC 821.

Some key aspects of the updated SMTP standard:

- Enhanced message size limits and error handling

- Support for international text through MIME extensions

- Introduction of the `ENHANCEDSTATUSCODES` keyword

- Formal definition of SMTP-TLS protocol for secure transfer

- Authentication mechanisms like LOGIN, PLAIN over TLS

So RFC 5321 gave us the enhanced SMTP protocol we use today - with security, authentication, and multimedia content support while preserving backward compatibility.

Difference between SMTP and ESMTP

- SMTP - Original simple mail transfer protocol with basic functionality

- ESMTP - Extended SMTP that adds various improvements

Some key differences:


|- |- |

|Plain text transmission|Supports TLS encryption |

|No authentication |Allows authentication |

|ASCII only message |Allows multimedia content |

|Limited command set |Enhanced commands and feedback|

|No delivery receipts |Delivery status notifications |

So essentially, ESMTP retains core SMTP functionality while incorporating various extensions for security and message handling. Most email servers today support ESMTP or at least a subset of SMTP extensions.

The road ahead

As email communication continues to grow globally, we can expect more evolution of SMTP to enhance reliability, security, and performance. Some possibilities for the future include:

- Mandating TLS encryption and authentication

- Reducing number of hops through MX optimization

- Support for large attachments and messages

- Native integration with identity protocols

- Using machine learning to improve deliverability

While alternatives like JSON/HTTP APIs exist, good old SMTP is here to stay given how deeply entrenched it is. But it will continue adapting to ensure your emails keep flowing seamlessly for decades to come.

Technical Aspects of SMTP

Now that we've seen SMTP's history, let's dive deeper into the tech behind how this protocol actually transfers your emails across the internet seamlessly.

Communication modes - End-to-end, Store and forward

SMTP supports two modes of communication - **end-to-end** and **store-and-forward**:

- End-to-end - The sending SMTP client communicates directly with receiving SMTP server. Email is transferred in one go.

- Store-and-forward - Emails go through multiple hops. Each SMTP server relays it to the next until destination is reached.

The store-and-forward approach is more common as it handles routing dynamically based on the domain names. Direct end-to-end transfer is rarely used.

Key commands

These commands primarily drive the SMTP transactions:

- HELO - Initiates session by introducing domain name

- MAIL FROM - Sets sender email address

- RCPT TO - Provides recipient address

- DATA - Sends actual message content

- QUIT - Closes the SMTP connection

There are also additional commands like `RSET`, `NOOP`, `HELP` etc. that handle other actions such as aborting, pinging the server and getting info.

Here's a typical command/response flow:


S: 220 mail.example.com ESMTP service ready

C: HELO client.example.org

S: 250 Hello client.example.org, pleased to meet you

C: MAIL FROM: [email protected]

S: 250 [email protected] OK

C: RCPT TO: [email protected]

S: 250 [email protected] OK


S: 354 Start mail input; end with .

C: From: [email protected]

C: To: [email protected]

C: Subject: Test message


C: Hello, this is a test SMTP email.

C: .

S: 250 Message accepted for delivery


S: 221 Bye, closing connection


Email message format

SMTP transports messages with a standard header and body format:

- Header - Contains metadata like sender, recipients, subject

- Body - The actual email content

- Null line - Blank line separating header and body

The receiving SMTP server reconstructs the email from this raw data before storing it for access.

Integration with POP3 and IMAP

SMTP handles only the sending of emails. To access received emails, protocols like POP3 and IMAP are required:

- POP3 - Downloads email to the client for offline access

- IMAP - Accesses email on server and keeps synchronized across devices

So a typical setup involves using SMTP to send mails and POP3/IMAP to receive them. This is why your email client needs to be configured with both sending (SMTP) and receiving (POP/IMAP) servers to function correctly.

Together, they form the fundamental building blocks of internet email. SMTP pushes emails across to your mailbox while POP3/IMAP enables you to pull mails from it for reading.

So in summary, SMTP provides a universal interserver mechanism for routing emails. Understanding its mode of operation, commands, and integration with other protocols is key to troubleshooting email issues.

SMTP Ports and Security

The exchange of myriad emails over the public internet requires robust security measures for privacy and integrity. Let's explore the vital role ports and protocols play in keeping your SMTP connections secure.

Standard port 25 and history

Since the beginning, **port 25** has been the standard port designated for SMTP traffic. Back in the 1980s when RFC 821 was drafted, security was an afterthought. SMTP communication was unencrypted plaintext that could be intercepted by anyone enroute.

But over time as the sensitivity of email data grew, traffic needed to be secured. This led to the creation of protocols like SSL and TLS specifically for encrypting application layer traffic including SMTP.

Ports 465 and 587 for SMTPS

To enable encrypted SMTP, new non-standard ports were defined:

- Port 465 - For legacy SMTP over SSL/TLS (SMTPS)

- Port 587 - For SMTP with STARTTLS command

Port 25 continues to be used for plain text SMTP in some cases, but it is considered insecure for most uses today. Port 587 with STARTTLS is the recommended approach for secure email delivery.

Encryption using TLS/SSL

Transport Layer Security (TLS) encrypts the SMTP session between hops to prevent interception or tampering. The handshake ensures authenticity and privacy:

- Server certificates validate identity

- Session keys encrypt the communication

- HMACs guarantee message integrity

Earlier, SMTP over SSL (SMTPS) was used but SSL is now deprecated in favor of more secure TLS standards.

Authentication and integrity checks

SMTP extensions also allow hosts to authenticate each other before transferring emails. Common authentication mechanisms include:

- LOGIN - Authenticate using username and password

- PLAIN - Send credentials unencrypted after TLS negotiated

- CRAM-MD5 - Challenge-response method based on hashes

Authenticating SMTP servers prevents spoofing of trusted identities for phishing. Digital signatures and hashes additionally protect message integrity in transit.

Port 2525 as an alternate

Some providers allow SMTP over **port 2525** as an alternate when earlier ports are inaccessible due to blocks by ISPs or infrastructure issues. However, this non-standard port does not guarantee security or authenticity.

Email spoofing and attacks

Despite defenses through TLS, authentication, and edge filtering, SMTP remains vulnerable to sophisticated spoofing and phishing attacks. Some ways emails can be forged or intercepted include:

- Email header manipulation

- Compromised credentials

- Rogue SMTP servers

- Unencrypted hops

- DNS cache poisoning

- Bugs in server software

So holistic protection requires a layered approach covering the entire email delivery chain.

In summary, SMTP security relies crucially on picky server configurations, vigilant traffic monitoring, and user awareness - not just protocols. As email expands globally, its underlying plumbing must evolve continuously to prevent mass-scale fraud.

Configuring and Troubleshooting SMTP

With SMTP being critical for email delivery, it's crucial to set it up correctly and troubleshoot issues promptly. Let's go over some key steps for smooth SMTP operations.

Setting up SMTP server locally vs using SMTP services

For sending outgoing mail, you can either:

- Host your own SMTP server - Lets you fully control configuration but requires expertise to install, secure and maintain a mail server stack like Sendmail, Postfix etc.

- Use an SMTP service provider - Easy to set up as the provider takes care of security, deliverability etc. But you're tied to their service limits and uptime.

Some popular SMTP services include SendGrid, Amazon SES, Mailgun etc. They offer APIs/libraries that apps can use for transmission.

Testing and troubleshooting connections

To diagnosing SMTP problems, tools like `telnet` and `openssl` prove handy:

1. Verify basic TCP connectivity


$ telnet mail.example.com 25


This checks if you can reach the SMTP server. Replace with your provider's domain.

2. Observe SMTP conversation


$ openssl s_client -connect mail.example.com:465 -crlf


OpenSSL allows seeing the SMTP handshake, TLS negotiation, and any errors returned.

3. Test commands

You can also manually send SMTP commands through Telnet or OpenSSL to isolate issues.

Debugging errors and problems

Some common SMTP errors and fixes include:

- Connection timeouts - Network issues, firewall blocking traffic, DNS failure

- TLS handshake failures - Certificates not trusted, protocol mismatch

- Authentication problems - Invalid credentials, mechanism not supported

- Rejected recipients - Typos in email address, greylisting in effect

- Message rejected - Attachment size limits, content filtering in place

- Delivery delays - Server unavailable, greylisting, spam filtering

Checking logs on SMTP servers also helps pinpoint where things are breaking down.

Security best practices for SMTP

Some ways to keep SMTP connections secure:

- Enforce TLS encryption between hops

- Require client and server authentication

- Filter traffic on ports 25,465,587 to block spam/attacks

- Monitor for unusual activity spikes indicative of attacks

- Keep mail server software patched and updated

- Use DNSSEC to prevent domain spoofing

- Implement DMARC, DKIM and SPF to authenticate emails

- Sandbox in-transit messages to isolate threats

- Educate users on phishing and social engineering

With email volumes growing exponentially, having robust visibility into SMTP traffic and rap2sponding quickly to anomalies is key to keeping inboxes threat-free.

In summary, hardening SMTP comes down to vigilance around security essentials - encrypt traffic, authenticate identities, filter judiciously, monitor closely and spread awareness. Keeping these fundamentals in check will go a long way in preventing nasty SMTP surprises.