Email Service (Gmail) — Simple

Problem statement (interviewer prompt)

Design a Gmail-scale email service: receive via SMTP, store per-user mailboxes durably, deliver outbound via SMTP, support folders/labels, full-text search, threading, attachments, IMAP/POP, and aggressive spam + phishing detection.

flowchart LR
  S([Sender MTA])
  RX[Inbound MTA<br/>SMTP - Postfix]
  AS[Anti-spam / DKIM / SPF / DMARC]
  STORE[(Mailbox Store)]
  IDX[(Search Index)]
  U([User])
  WEB([Web / IMAP / POP])
  S --> RX --> AS --> STORE
  STORE --> IDX
  U --> WEB --> STORE
  U --> WEB --> IDX
  WEB --> SMTPOUT[Outbound MTA] --> Internet

    classDef client fill:#dbeafe,stroke:#1e40af,stroke-width:1px,color:#0f172a;
    classDef edge fill:#cffafe,stroke:#0e7490,stroke-width:1px,color:#0f172a;
    classDef service fill:#fef3c7,stroke:#92400e,stroke-width:1px,color:#0f172a;
    classDef datastore fill:#fee2e2,stroke:#991b1b,stroke-width:1px,color:#0f172a;
    classDef cache fill:#fed7aa,stroke:#9a3412,stroke-width:1px,color:#0f172a;
    classDef queue fill:#ede9fe,stroke:#5b21b6,stroke-width:1px,color:#0f172a;
    classDef compute fill:#d1fae5,stroke:#065f46,stroke-width:1px,color:#0f172a;
    classDef storage fill:#e5e7eb,stroke:#374151,stroke-width:1px,color:#0f172a;
    classDef external fill:#fce7f3,stroke:#9d174d,stroke-width:1px,color:#0f172a;
    classDef obs fill:#f3e8ff,stroke:#6b21a8,stroke-width:1px,color:#0f172a;
    class S,U,WEB client;
    class RX,AS,SMTPOUT service;
    class STORE,IDX datastore;