How does email work?
Tags: webdev, interview, deeplearning, learning
Published: 2024-07-05 09:22:19

## In many interviews, one common question is, "Do you know how email works?" 

> Understanding this process is crucial, and here's a summary of how it functions. A lot happens when you hit "Send":


1. **Composition:**
 You write an email using an email client (like Gmail or Outlook).

2. **Assembling:**
 The email client combines the message body with the header (recipient, subject, date).

3. **Protocols:**
 Email uses protocols (SMTP, IMAP, POP) to reach the destination.

4. **Types of Clients:**
 Email clients are either web-based (e.g., Gmail) or client-based (e.g., Outlook).



## Email Composition and Sending

**Composition:**

- Emails are written using an email client (e.g., Gmail, Outlook).

- The client combines the message body with the header (recipient, subject, date).

**Sending:**

- Upon hitting send, the email is transmitted to an SMTP server.

- SMTP (Simple Mail Transfer Protocol) is responsible for handling outgoing mail.


## Email Transfer Process

**SMTP Server Processing:**

- The SMTP server identifies the recipient’s domain and performs a DNS (Domain Name System) lookup to find the IP address of the recipient’s mail server.

- It checks the MX (Mail Exchange) record to determine where to send the email.


**Email Transfer:**

- The email travels through multiple servers until it reaches the recipient’s SMTP server.

- If servers are busy or down, the email may be queued for later delivery.


## Email Reception and Retrieval

**Reception:**

- The recipient’s SMTP server receives the email and stores it in their mailbox.

**Retrieval:**

- The recipient accesses the email using an email client, either via POP3 (Post Office Protocol) or IMAP (Internet Message Access Protocol).

- IMAP allows emails to be accessed from multiple devices while remaining on the server.

- POP3 typically downloads the email to a single device, possibly removing it from the server.



![Image description](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/ozm5elhtp4ri34j5ekqq.png)


## Email Structure:

- **Envelope:** Contains sender and recipient addresses.

- **Body:** The actual message content, including text, images, and attachments.

- **Header:** Includes essential details like sender, recipient, date, and additional routing information (e.g., the path taken by the email).



## Protocols:

- **SMTP:** Used to send emails.

- **IMAP:** Accesses and syncs emails from the server.

- **POP3:** Downloads emails from the server.



## IMAP vs. POP3: A Simple Comparison


**IMAP (Internet Message Access Protocol):**

- Synchronization: Emails stay on the server and can be accessed from multiple devices.

- Two-Way Communication: Any changes (e.g., reading, deleting) sync across all devices.

- Ideal For: Users who access their email from various devices (phones, tablets, computers).


**POP3 (Post Office Protocol):**

- Download and Delete: Emails are downloaded to one device and usually deleted from the server.

- One-Way Communication: Actions on one device don't sync with others.

- Ideal For: Users who access their email from a single device and need offline access.

In essence, IMAP is more flexible and suitable for multiple devices, while POP3 is simpler and suited for single-device use.



## Types of Mail User Agents (MUA):

- **Email Clients:** Installed software like Outlook, Thunderbird.


- **Webmail:** Accessed through browsers like Gmail, Yahoo Mail.



## Email Bounces/ Undeliverable Messages:


**Permanent Errors (5xx errors):**

- Invalid Domain: Domain doesn't exist. Example: abc@yordomain.com instead of abc@yourdomain.com. Solution: Correct the domain name.

- No MX Records: No mail exchange records found. Solution: Contact recipient’s admin.

- Invalid Recipients: Incorrect or non-existent email address. Example: abb@yourdomain.com instead of abc@yourdomain.com. Solution: Check for typos.

- Email Policy Violation: Email violates recipient server policies. Example: Sending a .mov attachment when not allowed. Solution: Adhere to policies.

**Temporary Errors (4xx errors):**

- Server Busy: Recipient server is temporarily unavailable. Solution: Wait and retry.

- Greylisting: Temporary rejection for new or sudden email sources. Solution: Retry later.

- Too Many Emails: Sudden surge causes temporary rejection. Solution: Reduce email volume.

- Other Reasons: Full mailbox or anti-spam settings. Solution: Ensure compliance with recipient policies.



## Summary of Email Journey:

1. **Send:** You hit send, and the email goes to your SMTP server.

2. **SMTP Server:** The server looks up the recipient’s domain and gets the IP address via DNS.

3. **Transfer:** The email travels through several servers to reach the recipient's MTA(Mail Transfer Agent) server.

4. **Reception:** The recipient’s MTA(Mail Transfer Agent) server stores the email until accessed by the recipient.

5. **Retrieval:** The recipient uses an email client to access the email, either via POP or IMAP.


**Thank You**
Hope this helps. Your feedback is appreciated. Please feel free to correct any inaccuracies or provide additional information as needed.