Mail Protocols

SMTP (Simple Mail Transfer Protocol)

Purpose: SMTP is used to send emails from a client (e.g., Outlook, Gmail) to an email server and between email servers. It does not handle receiving emails.

Key Features:

Works on a push model (sends emails).
Used by mail servers to relay messages.
Requires authentication to prevent spam.
Supports encryption (TLS/SSL) for security.

SMTP Ports:

Port	Encryption	Usage
25	❌ None	Used for server-to-server email transfer (often blocked by ISPs).
465	✅ SSL	Deprecated but still in use.
587	✅ STARTTLS	Recommended for secure email sending.
2525	✅ STARTTLS	Alternative if 587 is blocked.

SMTP Commands:

Command	Description
`HELO` / `EHLO`	Initiates communication.
`MAIL FROM:`	Specifies the sender’s email.
`RCPT TO:`	Specifies the recipient’s email.
`DATA`	Starts the email content transmission.
`QUIT`	Ends the session.

SMTP Process:

Client connects to SMTP server on port 587 (or another port).
Server authenticates the client (if required).
Email is sent using SMTP commands (MAIL FROM, RCPT TO, DATA).
Server relays the email to the recipient’s mail server.
Recipient retrieves email via IMAP or POP3.

Security Measures:

Use STARTTLS (port 587) or SSL (port 465) for encryption.
Enable SMTP Authentication (SMTP AUTH) to prevent spam.
Use DKIM, SPF, and DMARC to prevent email spoofing.

POP3 (Post Office Protocol v3)

Purpose: POP3 is used for retrieving emails from a mail server to a client. It downloads emails and typically deletes them from the server, making it useful for offline access.

Key Features:

Works on a pull model (fetches emails).
Stores emails locally on the user’s device.
Does not sync emails across multiple devices.
Simpler and faster than IMAP but less flexible.

POP3 Ports:

Port	Encryption	Usage
110	❌ None	Default POP3 (unencrypted).
995	✅ SSL/TLS	Secure POP3 (recommended).

POP3 Commands:

Command	Description
`USER`	Sends the username.
`PASS`	Sends the password.
`STAT`	Checks the number of emails.
`LIST`	Lists emails in the mailbox.
`RETR`	Retrieves a specific email.
`DELE`	Deletes a specific email.
`QUIT`	Ends the session.

POP3 Process:

Client connects to the POP3 server on port 110 or 995.
User logs in using their credentials.
Server lists available emails.
Client downloads emails (and optionally deletes them from the server).
Connection closes, and emails are stored locally.

Limitations of POP3:

Emails do not sync across multiple devices.
Once downloaded, emails are removed from the server (unless configured otherwise).
Lacks features like folder organization (unlike IMAP).

Use IMAP instead of POP3 if you need multi-device email access.

IMAP4 (Internet Message Access Protocol v4)

Purpose: IMAP4 is used to retrieve and manage emails on a mail server while keeping them stored there. It allows users to access emails from multiple devices and syncs changes across them.

Key Features:

Emails remain on the server until deleted.
Supports multiple device access (syncs across devices).
Allows organizing emails into folders.
Supports partial downloads (view subject before full download).

IMAP4 Ports:

Port	Encryption	Usage
143	❌ None	Default IMAP (unencrypted).
993	✅ SSL/TLS	Secure IMAP (recommended).

IMAP4 Commands:

Command	Description
`LOGIN`	Authenticates the user.
`SELECT`	Selects a mailbox (e.g., Inbox).
`FETCH`	Retrieves email headers or body.
`STORE`	Marks messages (read, unread, deleted, etc.).
`EXPUNGE`	Deletes marked messages permanently.
`LOGOUT`	Ends the session.

IMAP4 Process:

Client connects to the IMAP server on port 143 or 993.
User logs in with credentials.
Server fetches email headers (without downloading full emails).
User reads emails (which remain on the server).
Changes (read, delete, move) sync across devices.

Advantages of IMAP4 Over POP3:

✅ Emails stay on the server (ideal for webmail & multi-device access).
✅ Syncs changes (read/unread status, folder organization).
✅ Supports remote email management (without full downloads).

Use IMAP4 if you need access from multiple devices, and use POP3 only if you want local storage with no syncing.

Understanding Universal Use case of SMTP and IMAP/POP working together.

Using SMTP with IMAP or POP3

SMTP, IMAP, and POP3 are used together for sending and receiving emails.

SMTP (Simple Mail Transfer Protocol) → Sends emails
IMAP (Internet Message Access Protocol) → Syncs and manages received emails
POP3 (Post Office Protocol v3) → Downloads received emails for local storage

How They Work Together

1️⃣ Sending Emails (SMTP)
- The email client (e.g., Outlook, Gmail) connects to an SMTP server.
- The user logs in and sends the email via the MAIL FROM, RCPT TO, and DATA commands.
- The SMTP server relays the email to the recipient’s email server.
- The recipient’s email server stores the email in the mailbox.

2️⃣ Receiving Emails (IMAP or POP3)
- The recipient’s email client connects to their mail server using IMAP or POP3.
- If using IMAP: The client syncs with the server, keeping emails on the server and allowing access from multiple devices.
- If using POP3: The client downloads emails and (by default) removes them from the server, making them available only on one device.

Example Setup in an Email Client

Function	Protocol	Port (Non-SSL)	Port (SSL/TLS)
Sending Emails	SMTP	25 / 587	465
Receiving Emails	IMAP	143	993
Receiving Emails	POP3	110	995

Choosing Between IMAP and POP3

Feature	IMAP	POP3
Email Storage	Stays on the server	Downloads to device
Access from Multiple Devices	✅ Yes	❌ No
Syncing Read/Unread Status	✅ Yes	❌ No
Local Email Backup Needed	❌ No	✅ Yes
Best For	Webmail & multiple devices	Single device, offline access

Practical Example

John uses Outlook.
SMTP (Port 587) sends emails from his Outlook to Gmail’s SMTP server.
IMAP (Port 993) syncs his inbox with Gmail’s server so he can check emails from his laptop and phone.

If John had used POP3, emails would be downloaded and removed from Gmail’s server, meaning they wouldn’t sync across devices.

IMAP + SMTP = Best for multi-device access.
POP3 + SMTP = Best for single-device offline access.

Differences Between IMAP and POP

### Impact of IMAP and POP3 on File Directories (Mailbox Structure)

IMAP and POP3 handle email storage differently, affecting how emails are stored, accessed, and organized on both the server and local machine.

IMAP’s Impact on File Directories

📌 Emails Stay on the Server
- The email client syncs with the server; emails are not permanently downloaded.
- The same folder structure is maintained across devices.

📌 Folder Structure is Preserved
- IMAP supports server-side folders (Inbox, Sent, Drafts, Spam, Custom Folders).
- These folders are available across all email apps & devices.

📌 Read/Unread Status Syncs
- If you read an email on one device, it is marked as read everywhere.

📌 Local Storage Impact
- No permanent local storage unless manually downloaded.
- Some clients cache recent emails for offline access.

📌 Example Directory Structure for IMAP:

/server/mailbox/user/
  ├── Inbox/
  ├── Sent/
  ├── Drafts/
  ├── Trash/
  ├── Spam/
  ├── CustomFolder1/
  ├── CustomFolder2/

POP3’s Impact on File Directories

📌 Emails are Downloaded & Removed from Server
- Emails are transferred from the server to the local machine.
- Once downloaded, they are deleted from the server (unless configured otherwise).

📌 No Folder Synchronization
- POP3 does not sync folders (Inbox, Sent, etc.).
- Each email app manages its own local folder structure separately.

📌 Requires an Email App for File Handling
- POP3 relies on the email client (e.g., Outlook, Thunderbird) to store and manage emails.
- The app must create local directories to organize emails.

📌 Local Storage Impact
- Emails are stored in local directories on the user’s device.
- If the computer crashes, emails can be lost unless backed up.

📌 Example Directory Structure for POP3:

/local/mail/
  ├── Inbox/  (Downloaded emails)
  ├── Sent/  (Managed by email app, not server-synced)
  ├── Archive/ (If user saves emails)
  ├── OldEmails.mbox (Static files storing past messages)

Comparison: IMAP vs. POP3 on Directories

Feature	IMAP	POP3
Where are emails stored?	On the server	On the local device
Folder structure?	✅ Syncs across devices	❌ Only local, no sync
Read/unread sync?	✅ Yes	❌ No
Requires an email app for file storage?	❌ No (server handles)	✅ Yes (app manages files)
Access from multiple devices?	✅ Yes	❌ No
Risk of losing emails?	❌ Low (stored on server)	✅ High (if not backed up)

IMAP keeps emails on the server, syncing across devices with folder support.
POP3 depends on an email app to store and manage emails locally.