Inbound Email Processing Checklist for SaaS Platforms

List the workflows that depend on inbound email, then set latency and durability targets that match customer expectations. Align requirements with product tiers so you can scale cost and reliability with usage.

Select webhooks for near real-time processing, add polling for backfill and failover. A hybrid approach helps during outages and supports reprocessing workflows.

Map how your app will generate and route unique email addresses that tie to tenants, users, or threads. Plus addressing and subdomains enable contextual routing without creating infinite mailboxes.

Store each message with normalized headers, a MIME part tree, and attachment metadata so downstream services can query reliably. Include indexes for tenant, recipient, and timestamps.

Use Message-ID, a canonical body hash, and envelope data to detect duplicates. Ensure consumers treat re-delivered events as safe to replay.

Create a separate domain or subdomain for inbound traffic with correct MX records and TLS. Isolation improves deliverability, routing clarity, and incident containment.

Publish SPF, sign DKIM, and enforce DMARC to protect your domain reputation and improve trust. Set up DMARC reporting to monitor spoofing attempts and policy impact.

Map the RCPT TO or Delivered-To fields to tenant and user context in a routing table. Reject unknown recipients early to reduce spam processing cost.

Enable addresses like tenant123@inbound.example.com or support+ticket-abc@example.com. Token patterns should be verifiable and reversible to avoid misroutes.

Use Variable Envelope Return Path to correlate bounces to the original recipient. Feed bounce data into rate limits, suppression lists, and billing signals.

Support multipart/alternative, multipart/mixed, and nested multiparts with boundaries. Preserve each part's headers to maintain fidelity for downstream rendering.

Handle quoted-printable, base64, and uncommon charsets, then normalize text to UTF-8. Incorrect decoding leads to data loss and search failures.

Convert HTML bodies to canonical plain text while stripping scripts, trackers, and unsafe tags. Maintain lists and quoted sections so replies parse cleanly.

Avoid loading large files into memory, compute a content hash, and store MIME metadata like filename, content-type, and size. Return stable URLs to your application layer.

Detect Content-ID attachments and map them to accessible URLs for rendering. Rewriting ensures in-app viewers display messages as intended.

Store DKIM-Signature, Authentication-Results, and Received headers for audits and fraud analysis. Keep the original RFC 5322 source for forensic needs.

Run AV scans and verify mimetype against file signatures to prevent abuse. Quarantine suspicious files and tag the message for downstream handling.

Apply row-level security so only the right tenant can access message content, attachments, and events. Audit every read and write of sensitive fields.

Detect credit cards, government IDs, API keys, and tokens in body text or attachments. Redact or tokenize before storing, then route originals to a secure vault if required.

Use a KMS with key rotation and separation by tenant to reduce blast radius. Log all key operations to meet SOC 2 and ISO 27001 controls.

Set default retention windows by plan, enforce purge jobs, and support tenant-driven legal holds. Include mechanisms to export or delete data for GDPR requests.

Attach a signature header using a shared secret, then verify and check for replay. Rotate secrets periodically and version your header scheme.

Retry transient failures, cap max attempts, and add jitter to avoid thundering herds. Respect 4xx vs 5xx semantics to prevent futile retries.

Include a stable event ID and compute a message hash so processing can be retried safely. Use transactional outbox patterns when updating your database.

Capture permanently failed events and provide a dashboard or CLI to inspect, fix, and reprocess. Tag with tenant IDs to prioritize high-value customers.

Publish structured events for messages received, bytes processed, and attachments stored. These power dashboards, usage-based billing, and anomaly detection.

Send seeded emails hourly with known content and assert end-to-end delivery, parsing, and webhook processing. Alert on latency and body mismatches.

Collect samples that include quirky clients, edge charsets, and broken MIME boundaries. Use them in regression tests to prevent decoding regressions.

Measure delivery latency, parse failure rate, webhook success rate, message sizes, and spam score distribution. Tie alerts to SLO burn rates for faster response.

Alert on spikes in DMARC failures, oversized attachments, or sudden tenant-specific volume. Include context like tenant ID, region, and sample Message-ID.

Document steps to replay events, restore from backups, and purge data by tenant. Include on-call checklists and escalation paths for rapid resolution.