Integrate Multi-Provider Email Service

Problem

Design and implement an email sending service that uses multiple email providers (SendGrid, AWS SES, Postmark) with automatic failover. If the primary provider is down or rate-limited, the system should transparently switch to a backup provider without losing any emails.

Requirements

1. Multi-Provider Support: Integrate with at least 3 email providers behind a unified interface.
2. Automatic Failover: If the primary provider fails, automatically route emails through a backup provider.
3. Circuit Breaker: Implement a circuit breaker per provider. After N consecutive failures (or >50% failure rate in a window), "open" the circuit and stop sending to that provider.
4. Priority Routing: Route transactional emails (password reset, order confirmation) through the fastest/most reliable provider. Route marketing emails through the cheapest provider.
5. Delivery Tracking: Track the delivery status of each email across providers using webhooks from each provider.
6. Rate Limiting: Respect each provider's rate limits. Distribute volume across providers when approaching limits.
7. Template Abstraction: Define email templates once and render them for any provider (each provider has a different template format).

Constraints

• The system sends 50,000 emails per day.
• Transactional emails must be sent within 5 seconds of being triggered.
• Marketing emails can be batched and sent over hours.
• Provider failover must happen within 10 seconds of detecting a failure.
• No email should be lost, even during provider outages (at-least-once delivery).
• The system must comply with CAN-SPAM and GDPR (unsubscribe links, consent tracking).

What to Design

• The provider abstraction layer and interface
• The circuit breaker implementation with state machine
• The failover and provider selection algorithm
• The delivery tracking unification across providers
• The template rendering pipeline