How SMTP Works: The Journey of an Email Message

When you hit “Send” in your inbox, a fascinating journey begins across the Internet.

Your message doesn’t travel directly to the recipient — it passes through several servers, protocols, and validation steps to ensure it arrives intact and unaltered.

The main actor in this process is SMTP (Simple Mail Transfer Protocol), the protocol that has powered global email delivery since the 1980s.

What is SMTP

SMTP is the standard that defines how email messages are transmitted between servers over the Internet. Originally defined in RFC 821 (1982) and later updated by RFC 5321, it remains the foundation of all email delivery systems.

Its role is simple but essential: to deliver messages from the sender’s mail client to the recipient’s mail server, following a structured sequence of commands and responses.

The journey of an email, step by step

Sending an email is not a simple transfer — it involves multiple components and protocols working together:

1. Mail Client (MUA – Mail User Agent)
   This is the application you use to send the message (Outlook, Gmail, Thunderbird, etc.). The client connects to the sender’s SMTP server and hands over the message for delivery.

2. Outgoing Server (MTA – Mail Transfer Agent)
   The SMTP server analyzes the recipient’s address and queries the DNS to determine which server is responsible for the destination domain.

3. MX Record Lookup
   The DNS (Domain Name System) identifies the mail server that manages incoming mail for the recipient’s domain. For instance, for user@company.com, the MX record may point to mail.company.com.

4. Message Transmission
   The sending server connects to the recipient’s server and performs an SMTP conversation using commands like HELO, MAIL FROM, RCPT TO, and DATA.
   If the delivery succeeds, the recipient’s server replies with code 250 OK.

5. Incoming Server (MDA – Mail Delivery Agent)
   Once received, the message is stored in the recipient’s mailbox and made available via other protocols such as IMAP or POP3.

6. Message Reception
   The recipient opens their email client and downloads or syncs messages from the destination server.

Example of an SMTP Dialogue

To better understand the process, here’s a simplified example of a real SMTP conversation between the sender’s server and the recipient at johndoe@example.com:

S represents the Server and C the Client.

This exchange of commands and responses happens every time an email is sent. The receiving server may accept, reject, or defer delivery depending on domain policies and system status.

Protocols that work alongside SMTP

Email doesn’t rely on a single protocol. SMTP works together with others that complete the communication cycle:

• IMAP (Internet Message Access Protocol) – allows you to read messages directly from the server and keep them synced across devices.
• POP3 (Post Office Protocol v3) – downloads messages to the user’s device, usually removing them from the server.
• MIME (Multipurpose Internet Mail Extensions) – defines how to send attachments, images, and rich content such as HTML or PDF files.

Security and authentication

SMTP was designed in a more “innocent” Internet era, without encryption or authentication. Today, it is secured through several layers of protection:

• STARTTLS – encrypts communication between servers using TLS. This is the encryption method implemented by eEvidence by default.
• SPF, DKIM, and DMARC – validate the sender’s authenticity and protect against spoofing attempts.
• SMTP Authentication (AUTH) – ensures only authorized users can send emails from a domain.

These elements are essential for deliverability and domain reputation, especially for business or registered communications.

Common SMTP errors and codes

During transmission, servers exchange numeric codes indicating the status of delivery:

These codes help diagnose why a message fails to reach its destination.

SMTP and Registered Email

In registered email services such as eEvidence, SMTP plays a crucial role.

The system records every step of the protocol — from connection to delivery or rejection — and generates technical evidence (traces, timestamps, digital signatures) that serve as legally admissible proof of sending, content, and delivery.

Through this level of control, registered email transforms a standard SMTP exchange into a legally valid communication.

Frequently Asked Questions (FAQ)

What’s the difference between SMTP and IMAP?

SMTP is used to send messages, while IMAP is used to receive and manage them on the server.

Why do some emails fail to deliver?

It can be due to spam filters, DNS errors, authentication failures (SPF/DKIM/DMARC), or temporary rejections by the destination server.

Does SMTP encrypt messages?

Not by itself. Encryption is achieved via STARTTLS or TLS tunnels, which protect transmissions from interception.

Can I use SMTP with any domain?

Yes, as long as the server is authorized and correctly configured (SPF, DKIM, and DMARC records).

Conclusion

The SMTP protocol is the beating heart of email.

Although it was designed in the early days of the Internet, it remains the foundation on which billions of messages are sent every day.

Understanding how it works helps us appreciate the complexity and reliability of the system — and, in the case of eEvidence, how a technical process becomes legally valid proof of communication.