IMAP
Synchronizes folders, flags and server-side message state across devices.
C: A1 SELECT INBOX
S: * 18 EXISTS
C: A2 FETCH 18 BODY[]APPLICATION LAYER · STORE AND FORWARD
Travel with one email from Compose to Inbox.
Email is a federation of protocols, queues and trust signals—not one send operation. Explore submission, SMTP relay, DNS MX, MIME, delivery authentication and mailbox retrieval as connected sub-labs.
FIVE CONNECTED SUB-LABS
Change viewpoint without losing the same message
The simulator never sends real email. Addresses use reserved example domains.
Mail User Agentcompose + submit
aliceSubmission serverSMTP AUTH · 587
example.testDNS MXfind receiver
example.net MX?Receiving MTASMTP · 25
mail.example.netMailboxstore + IMAP
bob/INBOXREADY · Compose a message and start delivery.
IDLENo envelope has been submitted.
Envelope ≠ message headers. MAIL FROM and RCPT TO route delivery; From and To inside DATA are presentation fields and can differ.
Raw attachment0 B
Base64 payload0 B
Encoding overhead0%
POP3
A simpler download-oriented mailbox protocol with a small command model.
C: USER bob
C: PASS ******
C: RETR 18| Question | IMAP | POP3 |
|---|---|---|
| Server folders | First-class feature | Minimal |
| Multiple devices | Designed for synchronization | Possible but less expressive |
| Typical ports | 143 / TLS 993 | 110 / TLS 995 |
STORE, FORWARD, RETRY
A temporary failure is not a permanent rejection
2xx
Requested mail action accepted. Responsibility may have transferred to the next server.
4xx
Temporary failure. Keep the message in a queue and retry with backoff.
5xx
Permanent failure for this attempt; normally generate a delivery status notification.
CLASSROOM TASKS
Evidence across protocol boundaries
- Trace which component uses DNS and why it asks for MX rather than A first.
- Compare SMTP envelope addresses with message From/To headers.
- Measure Base64 overhead for several attachment sizes.
- Inject 451 and design a retry schedule that avoids overload.
- Find a case where SPF fails but aligned DKIM lets DMARC pass.
- Explain why end-to-end encryption is different from SMTP TLS.