Requirements

Functional Requirements

• Send and receive email (SMTP/IMAP/POP3)
• Inbox, sent, drafts, spam, trash folders
• Full-text search across email content
• Attachment support (up to 25 MB)
• Spam and phishing detection
• Email forwarding and auto-reply rules
• Read receipts and delivery status

Non-Functional Requirements

• Delivery: At-least-once delivery with exactly-once semantics
• Latency: Email delivered within 30 seconds
• Storage: 15 GB per user, 1.8 billion users
• Search: Full-text search in < 1 second
• Spam: Filter > 99% of spam emails
• Availability: 99.99%

Back-of-the-Envelope Estimation

API Design

POST /api/v1/messages/send
Request: {
  "to": ["user@example.com"],
  "subject": "Hello",
  "body": "Plain text or HTML",
  "attachments": ["file_id_123"]
}
Response: { "message_id": "msg_abc123", "status": "queued" }

GET /api/v1/messages?folder=inbox&limit=50
Response: { "messages": [...], "total": 1234 }

GET /api/v1/messages/msg_abc123
Response: { "message_id": "...", "from": "...", "body": "..." }

GET /api/v1/search?q=invoice+2026
Response: { "messages": [...], "count": 42 }

High-Level Architecture

Detailed Design

Email Delivery Flow

SMTP Protocol

S: 220 mail.example.com ESMTP
C: HELO sender.com
S: 250 Hello
C: MAIL FROM:<sender@sender.com>
S: 250 OK
C: RCPT TO:<recipient@example.com>
S: 250 OK
C: DATA
S: 354 Start mail input
C: Subject: Hello
C: From: sender@sender.com
C: To: recipient@example.com
C: 
C: This is the email body.
C: .
S: 250 OK: Message queued
C: QUIT

Spam Filtering

Email Storage

Store emails in a distributed storage system:

Delivery Guarantees

Practice Exercises

1. Design: How would you implement email forwarding rules (e.g., "forward emails from boss to phone")? Design the rules engine.

2. Scale: If Gmail stores 15 GB per user for 1.8 billion users, estimate the total storage needed and the distributed storage architecture.

3. Reliability: Design a system to guarantee exactly-once email delivery. What challenges arise from SMTP retries and network partitions?

4. Search: How would you implement full-text search across 100 billion emails? What indexing strategy would you use?

What to Learn Next