Email / SMS Delivery (Twilio-like) — Simple

Problem statement (interviewer prompt)

Design a Twilio-style transactional email + SMS delivery service. Accept API calls from customers, route through multiple carriers/providers, manage sender reputation, handle bounces + opt-outs + DLR receipts, and prove deliverability + auditability.

flowchart LR
  CUS([Customer])
  API[Send API]
  Q[[Queue]]
  DEL([Channel workers])
  SMTP((Email MTAs / SES))
  SMS[SMS gateways]
  CUS --> API --> Q --> DEL
  DEL --> SMTP
  DEL --> SMS

    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 CUS client;
    class API,SMS service;
    class Q queue;
    class DEL compute;
    class SMTP external;