Email Testing Checklist for SaaS Platforms

List the specific flows you support, such as support ticket replies, task creation from emails, or workflow automation triggers, and define how success is measured for each. Tie each to concrete assertions like fields extracted, thread linkage, and downstream side effects.

Document required fields like from, to, cc, subject, text, html, attachments, messageId, inReplyTo, and custom headers. Include schema versioning to support non-breaking upgrades.

Define maximum time from SMTP accept to parsed JSON available and webhook delivered, per environment. Align retry budgets with product expectations and on-call capacity.

Decide whether to encode tenant and resource IDs in the local-part (e.g., user+tenantId@) or subdomain per tenant (tenant.example.com). Document parsing rules and collision handling.

Identify sensitive patterns like API keys, passwords, and tokens and configure filters before persistence. Verify scrubbing applies to subject, headers, body, and attachment text.

Include Gmail, Outlook, Apple Mail, Yahoo, Exchange/Office 365, mobile clients, and forwarding services. Map each to MIME edge cases you need to cover.

Generate unique recipient addresses per test or pipeline, tagged by build ID, and auto-expire them. This prevents cross-test contamination and simplifies clean-up.

Point MX for dev, staging, and QA subdomains to sandbox processing and restrict production addresses to vetted flows. Add guardrails that block cross-environment forwards.

Send to user+ticket123@yourdomain and assert the parser extracts ticket123 reliably, even with URL-encoded or mixed-case tokens. Include tests for nested tokens like user+tenant42-thread77@.

Simulate bounces with return paths that embed identifiers and confirm your system correlates delivery failures without creating new items. Ensure loops are prevented.

Route unknown recipients in non-production to a quarantine mailbox and assert they are not processed into customer artifacts. Log and surface these events for triage.

Forward from real-looking domains to your sandbox address and confirm critical headers (Message-ID, In-Reply-To, References, List-Id) survive. Record provider-specific quirks.

Burst 100+ emails into a single tenant and assert backpressure, queue depth metrics, and webhook throughput behave as expected without data loss.

Test emails with both text/plain and text/html parts and assert the chosen content aligns with your rendering and security policy. Sanitize HTML or prefer text as required.

Send messages using quoted-printable, base64, and charsets like UTF-16 and Shift-JIS. Confirm correct Unicode output and error handling on malformed input.

Include forwarded messages and digests, ensuring nested messages are parsed or safely surfaced per your product needs. Validate that metadata does not overwrite parent context.

Map cid: references in HTML to extracted attachments and store stable references (e.g., SHA-256). Ensure missing or duplicate CIDs do not break rendering.

Feed inputs with truncated parts, missing Content-Type, or extra CRLFs and confirm graceful fallback. Ensure a clear error field is set without dropping the message.

Capture Message-ID, In-Reply-To, References, and List-Id consistently. Build deterministic threading keys and verify edge cases where clients rewrite headers.

Test 25-50 MB attachments and verify streaming, temporary storage quotas, antivirus hooks, and user-facing rejection messages when limits are exceeded.

Handle text/calendar and message/delivery-status parts appropriately, creating events or surfacing bounce metadata without polluting user workflows.

Validate signatures over the raw body using a shared secret and reject requests outside a short time window. Store minimal nonce or timestamp to defend against replays.

Derive a key from provider event ID or Message-ID plus recipient to de-duplicate at the database boundary. Assert duplicate webhook deliveries cause no side effects.

Retry 5xx and network errors using capped backoff, move permanently failed webhook events to a dead-letter queue, and expose replay tooling.

Cap webhook receiver timeouts to 2-5 seconds, treat 429 and 503 as retriable, and avoid retrying hard 4xx errors. Emit structured error reasons for analytics.

Include a schema_version field and keep parsers backward-compatible for at least one version. Add feature flags for new fields to enable staged deployments.

When webhooks are blocked, poll by since_id or timestamp with pagination. Verify partial failures do not cause skips or duplicates.

Do not assume any ordering guarantees across webhooks or polling. Make each event independently replayable and idempotent.

Send known fixtures into disposable addresses, assert webhook delivery and storage records, and page on breaches of latency SLOs.

Inject messages with spam-like content and known test patterns to ensure they are quarantined or flagged, not executed as automations.

Reply from multiple clients and via forwards to ensure correct conversation mapping and no duplicate tickets or comments are created.

Attempt cross-tenant access by guessing IDs or using misrouted addresses and confirm access is denied and audited.

Track time-to-parse, webhook success rate, queue depth, and attachment rejection rates. Set actionable thresholds and on-call runbooks.

Retain encrypted raw messages for a defined TTL and support parser re-runs with version tagging. Verify idempotent updates on replay.

Test GDPR/right-to-erasure flows for email-derived records and attachments, including search-index purges and storage lifecycle rules.