Postfix Sender-Dependent Routing

Operational objective

Routing messages by sender while preserving predictable fallback behavior.

The objective is not merely to make mail flow. A production design must remain observable, recoverable and safe when a provider throttles traffic, a credential is abused, DNS changes unexpectedly, or queue pressure rises.

Architecture and boundaries

Document the systems involved, trust boundaries, source addresses, sender identities, routing maps, provider policies and ownership. For Postfix operations, keep transactional, warm-up, marketing and customer traffic separable wherever practical.

• Define the authoritative configuration source.
• Identify every dependency and fallback.
• Separate control-plane changes from message injection.
• Record the rollback command before deployment.

Implementation workflow

1. Capture the current configuration and runtime state.
2. Build the change in a staging copy or isolated route.
3. Run syntax and dependency checks.
4. Deploy to a limited traffic segment.
5. Compare acceptance, deferral, bounce and queue-age metrics.
6. Expand only after the success criteria remain stable.

Validation checklist

• Identity: PTR, HELO, SPF, DKIM and DMARC are consistent.
• Security: relay and authentication boundaries are explicit.
• Routing: the intended source IP and destination policy are confirmed.
• Observability: logs contain queue ID, sender, recipient, route and remote response.
• Recovery: backups and rollback commands were tested.

Failure modes and diagnosis

Common failures include syntax that parses but routes incorrectly, stale map databases, unexpected DNS caching, provider-specific throttling, queue starvation, credential misuse and over-broad fallback. Diagnose by narrowing the failure across time, provider, source IP, sender domain, campaign class and message result.

# Evidence pattern
Timestamp | Queue ID | Source IP/VMTA | Sender | Recipient domain
Remote MX | SMTP code | Enhanced code | Response text | Retry count

Production recommendations

For Postfix Sender-Dependent Routing, prefer small reversible changes over broad emergency tuning. Do not increase concurrency or retry frequency merely because a queue is growing; first determine whether the limiting factor is local capacity, provider policy, DNS, TLS, identity or reputation.

Keep a short operational record containing the problem statement, evidence, change, expected result, actual result and next review time.