Design temporary email service
Overview
This question evaluates system design competency across distributed architecture, SMTP ingestion and mail pipelines, data modeling for ephemeral inboxes and attachments, scalability, security and abuse mitigation, observability, and operational trade-offs.
Confluent
Sep 6, 2025, 12:00 AM
Software Engineer
Technical Screen
System Design
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System Design: Disposable Email Service with Expiring Inboxes
Context
Design a production-grade disposable/temporary email service for web/mobile clients. Anonymous users can create randomized inbox addresses that auto-expire after a configurable TTL (e.g., 10–60 minutes). The service must receive messages via SMTP (including attachments) and expose a simple UI/API to view messages before expiry.
Requirements
Design the system to cover:
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High-level architecture
- DNS/MX records and Mail Transfer Agent (MTA)
- SMTP ingress and acceptance policy
- Message pipeline (parsing, scanning, storage)
- Storage layers (metadata and blobs)
- Data model and indexing for inboxes, messages, attachments
- Address generation, TTL, and cleanup/GC
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Spam filtering and abuse controls
- Rate limiting, blocklists, greylisting, CAPTCHAs/PoW
- Attachment handling and size limits
- Privacy and data retention policies
- Multi-tenant domains (e.g., multiple receiving domains/brands)
- Observability (metrics, logs, tracing, SLOs)
- Scaling and cost considerations
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Key APIs
- Create/refresh inbox, fetch messages/attachments
- Consistency and durability trade-offs
- Mitigations for hot inboxes and denial-of-service attacks
State assumptions if needed and justify key trade-offs.
Solution
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