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Protocol Explorer · SMTP

Explore internet protocols with animated messages, real arrival order and packet inspection.

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Mail Server198.51.100.25InternetRouter192.168.1.1Client192.168.1.10

Protocol overview

SMTP

Active

Simple Mail Transfer Protocol

Submit a message using a mail envelope and a DATA body. Distinguish SMTP delivery from the POP3 or IMAP protocols used to access a mailbox.

Layer
Application (Layer 7)
Default port
25 relay / 587 submission
Transport
TCP; often TLS
Data format
Text commands + message body
Defined in
RFC 5321, RFC 3207
Read the complete guide ↗

Packet flow

DNSTCPRequestResponse
No.TimeSourceDestinationProtocolInfoSize
10.000s198.51.100.25192.168.1.10SMTP220 mail.example.com ESMTP82 B
20.024s192.168.1.10198.51.100.25SMTPEHLO client.example.com79 B
30.048s198.51.100.25192.168.1.10SMTP250-mail.example.com90 B
40.072s192.168.1.10198.51.100.25SMTPSTARTTLS64 B
50.096s198.51.100.25192.168.1.10SMTP220 Ready to start TLS78 B
60.120s192.168.1.10198.51.100.25SMTPMAIL FROM:<[email protected]>86 B
70.144s198.51.100.25192.168.1.10SMTP250 OK62 B
80.168s192.168.1.10198.51.100.25SMTPRCPT TO:<[email protected]>84 B
90.192s198.51.100.25192.168.1.10SMTP250 OK62 B
100.216s192.168.1.10198.51.100.25SMTPDATA60 B
110.240s198.51.100.25192.168.1.10SMTP354 End with <CRLF>.<CRLF>82 B
120.264s192.168.1.10198.51.100.25SMTPFrom: [email protected]146 B
130.288s198.51.100.25192.168.1.10SMTP250 Queued66 B
140.312s192.168.1.10198.51.100.25SMTPQUIT60 B

NAVSPROUT · PROTOCOL FIELD GUIDES

SMTP, packet by packet.

Submit a message using a mail envelope and a DATA body. Distinguish SMTP delivery from the POP3 or IMAP protocols used to access a mailbox.

PROTOCOL FIELD GUIDE

How SMTP works

Submit a message using a mail envelope and a DATA body. Distinguish SMTP delivery from the POP3 or IMAP protocols used to access a mailbox.

Default port
25 relay / 587 submission
Transport / carrier
TCP; often TLS
Category
application

1. Greet the server

A server greeting starts the session. EHLO lets the client discover extensions such as STARTTLS and supported authentication mechanisms.

2. Protect the session

When STARTTLS succeeds, a TLS handshake follows and the client sends EHLO again. This lab shows the SMTP layer; real credentials should travel over protected submission.

3. Set envelope addresses

MAIL FROM sets the return path; RCPT TO specifies a recipient. These envelope addresses are distinct from visible From and To headers.

4. Transfer the message

After 354, the client sends headers and body, ending with CRLF-dot-CRLF. Lines beginning with a dot are escaped by dot-stuffing.

5. Accept responsibility

A successful 250 after DATA means the server accepted responsibility, not that a person has read the message. QUIT ends the SMTP session.

Protocol relationships

Protocols cooperate across layers. Compare the neighboring lessons to distinguish addressing, delivery, application semantics and encryption.

Standards and editorial method

Written and implemented independently by NavSprout. Lessons summarize public protocol standards using synthetic documentation addresses and fictional credentials. Consult the standards for complete wire formats and normative behavior.

RFC 5321 ↗ RFC 3207 ↗

Check your understanding

Which command specifies an envelope recipient?

Why it’s hereNAVSPROUT ORIGINAL

Reading definitions is easier when you can connect them to a packet. This first-party learning tool pairs static explanations with a visual lab, byte inspection and a short knowledge check. All course data and application resources ship with NavSprout.

  1. Choose DNS and follow the lookup from client to authoritative server.
  2. Switch to the sequence view, then inspect a packet and select a field to highlight its bytes.
  3. Compare TCP with UDP, answer the quiz and export your progress.

Frequently asked questions

What happens when I open a website?

A typical first HTTPS visit can involve DNS resolution, a transport connection, a TLS handshake and an HTTP exchange. Caches, connection reuse and HTTP/3 can change this sequence.

How are TCP and UDP different?

TCP presents a reliable ordered byte stream. UDP preserves individual datagram boundaries without built-in delivery guarantees. Applications choose the behavior they need.

Are these real captured packets?

No. Frames are generated locally for teaching. Header lengths and checksums are calculated, while TLS ciphertext and some negotiation payloads are illustrative. Downloaded PCAP files contain synthetic examples, not live traffic.

Can I learn without JavaScript or an account?

All 14 protocol guides and their lesson explanations are available as ordinary HTML without signing in. JavaScript powers animations, byte inspection, local progress and quizzes.

Where is my progress saved?

Only in this browser’s local storage. Export a progress JSON file to move it to another device. Clearing browser data can remove progress.

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