MIME Parsing Checklist for SaaS Platforms

Document which product flows depend on inbound email, expected daily message counts, and peak bursts so capacity and budget models match reality.

Decide between unique inbox per user, plus-addressing, or tagged aliases to reliably route each message to a tenant and principal without custom rules.

List must-have fields like envelope recipients, normalized headers, chosen body variant, and attachment metadata, and include a schema version for safe evolution.

Combine Message-ID, DKIM canonicalized body hash, and a content digest to prevent duplicate processing across retries and provider replays.

Define how aliases or subdomains resolve to tenant IDs and enforce access controls so one customer's mail never leaks across accounts.

Choose inline rendering rules, object storage locations, AV scanning points, and TTLs to balance user experience, security, and cost.

Use per-tenant or per-environment subdomains and point MX records to your receiving servers to isolate traffic and simplify routing.

Enable STARTTLS, support 8BITMIME, and set sane size limits so legitimate mail does not bounce while preventing resource abuse.

Use multiple MX hosts with proper priorities and health checks to survive regional outages without message loss.

Persist RCPT TO values alongside headers to ensure correct tenant routing even when To/Cc differ from the SMTP envelope.

Route unparseable messages and failed deliveries to durable buckets with metadata so they can be inspected and replayed safely.

Throttle by sender domain and target alias, and block high-risk patterns to protect shared capacity in a multi-tenant environment.

Parse MIME incrementally to avoid loading full messages into memory, protecting CPU and RAM under spikes and large attachments.

Support base64 and quoted-printable, detect mislabeled charsets, and normalize to UTF-8 while recording original encodings for traceability.

Choose how to prioritize text/plain over text/html or keep both, and document product-specific rules like stripping tracking pixels.

Rewrite cid: references to signed URLs or data placeholders and tag those parts as inline to avoid double counting in attachment lists.

Canonicalize From, To, Cc, Reply-To, Message-ID, and References, fold continuations, and guard against header duplication or injection.

Compute content digests, sanitize filenames, record MIME type, size, and disposition, and store hashes for integrity verification.

Include a signature header with algorithm, a UNIX timestamp, and nonce to allow recipients to verify authenticity and prevent replay.

Attach a stable event UUID and allow consumers to safely retry, storing last-seen IDs and status for each tenant endpoint.

Provide a reliable fallback for teams unable to expose webhooks, using time-window cursors and incremental acknowledgments.

Deliver compact JSON and expose short-lived links for EML and large attachments to reduce webhook size and improve reliability.

Expose SPF, DKIM, DMARC results and spam annotations so downstream systems can flag risky messages or adjust workflows.

Use exponential backoff, per-tenant concurrency caps, and circuit breaking to protect your system and customer endpoints during incidents.

Remove scripts, event handlers, and embedded forms, and neutralize URLs to prevent XSS when rendering user-supplied mail content.

Scan attachments on ingress and before download, and block dangerous types like executables or macro-enabled documents.

Use a KMS to protect stored EML and attachments, rotate keys, and log decrypt operations for strong tenant isolation.

Support configurable TTLs and an API to purge messages and attachments on demand for privacy requests and contract terms.

Record who accessed message bodies or attachments and why, and make logs tamper-evident for security and compliance reviews.

Attach region, storage class, and processing purpose to each message so downstream systems can enforce policy routing.

Include charsets like Shift-JIS and ISO-2022-JP, deeply nested multiparts, calendar invites, and winmail.dat to validate real-world cases.

Hammer the parser with broken boundaries, invalid headers, and oversized parts to detect crashes and ensure graceful degradation.

Regularly force 429 and 5xx responses, verify retries and idempotency, and confirm DLQ paths and manual replay tools work end to end.

Track ingress rate, parse time percentiles, body selection outcomes, attachment counts, webhook latency, and DLQ depth per tenant.

Page on sustained parse failures, signature verification errors, or elevated webhook 5xx by tenant to enable targeted response.

Run load tests that mirror marketing campaigns or billing cycles and confirm queues, rate limits, and autoscaling kick in safely.