Why Deliverability Is a Core Feature
A temporary email service lives or dies on whether verification and confirmation messages actually arrive. Deliverability is not magic—it’s engineering: DNS hygiene, protocol correctness, domain strategy, abuse controls, rendering safety, and reputation management. This deep dive shows how to achieve high inbox rates (93–96% on consumer providers in our lab) while keeping the platform privacy-first.
Deliverability KPIs to Track
- Inbox placement (by provider): Gmail, Outlook/Hotmail, Yahoo/AOL, plus top regional providers.
- Time-to-inbox: P50/P90 from SMTP accept to user-visible delivery.
- Bounce rate: Soft/hard bounce split; per-domain granularity.
- Block reasons: RBL hits, disposable-domain policies, malformed headers, SPF/DKIM/DMARC failures.
- Engagement proxies: Open/click are noisy for disposable inboxes; focus instead on “verification link seen” telemetry if privacy policy allows (prefer opt-in).
Domain and DNS Strategy
- SPF: Publish a minimal SPF record that authorizes only the systems that truly send mail for your domains. Example:
v=spf1 include:_spf.mailhost.example -all.
- DKIM: Sign outbound mail, even if you primarily receive. It signals hygiene to receivers and reduces spoofing when you send opt-in alerts or system notices.
- DMARC:
p=none with rua reports to start; graduate to p=quarantine once you confirm alignment. DMARC reporting gives early signals of spoof or delivery drift.
- MX diversity: Use at least two MX hosts across different availability zones/providers; keep TLS on with modern ciphers.
- PTR and A/AAAA correctness: Reverse DNS on your ingress MTAs should match forward DNS; many providers penalize mismatches.
Domain Pool Management
- Maintain a small, well-monitored pool of domains; retire domains that enter blocklists frequently.
- Avoid vanity or “spammy” terms in domains; choose neutral, short names.
- Stagger domain onboarding: warm up with low traffic, watch bounce patterns, then promote.
SMTP and Protocol Hygiene
- TLS everywhere: Offer STARTTLS with modern ciphers; avoid deprecated protocols.
- Correct headers: Ensure
Date, Message-ID, From, and To are valid and timezone-aware.
- No malformed MIME: Normalize charset to UTF-8; strip malformed parts; reject oversized attachments.
- Greylisting tolerance: Implement retry with exponential backoff; respect 4xx semantics; do not hammer remote MTAs.
- Sane limits: Cap message size (e.g., 10–20 MB); reject executable attachments or HTML with active content.
Anti-Abuse Without Hurting Delivery
- Creation throttles: Limit new inbox creation per IP/device to curb spam relays.
- Inbound rate limits: Cap messages per address per hour; drop obvious bulk floods.
- Heuristics for loops: Detect auto-replies and disable forwarding of potential mail loops.
- Block relays to other disposables: Prevent being a hop in a spam pipeline.
- List-unsubscribe respect: If you send system mail, include
List-Unsubscribe headers and honor requests.
Rendering and Safety
- Default to plain text: Render HTML as sanitized, with images blocked by default.
- Neutralize tracking pixels: Strip or rewrite
<img> to data-free placeholders unless user opts in.
- Disable scripts/styles: Remove
<script>/on* attributes; sanitize CSS that can exfiltrate (e.g., url()).
- Attachment handling: Quarantine executables; block dangerous MIME types; offer safe previews for text/image where possible.
Observability with Privacy
- No PII in logs: Hash mailbox identifiers; avoid logging subject/body.
- Metrics not payloads: Emit counters and histograms (deliveries, bounces, delays) without content.
- Sampling for debugging: If you must sample payloads, do it in a private, short-lived buffer with explicit opt-in and automated purge.
Deliverability Playbook (Step-by-Step)
- Baseline DNS
- Publish SPF/DKIM/DMARC.
- Verify MX health via
dig, mxtoolbox, and Gmail Postmaster Tools.
- Ingress MTA Hardening
- Enforce TLS, size limits, connection limits.
- Normalize headers and encodings; reject malformed mail early.
- Domain Rotation Rules
- Move traffic off a domain if hard bounces spike or you see RBL hits.
- Keep a “quarantine” pool for testing before re-promoting a domain.
- Provider-Specific Checks
- Gmail: align SPF/DKIM, avoid “From” spoofing, stable reverse DNS.
- Outlook: respect throttling, avoid rapid retries, stable HELO/EHLO.
- Yahoo: monitor Feedback Loop (FBL) if applicable; handle 4xx gracefully.
- Inbox Experience
- Fast indexing: messages should appear within seconds of SMTP accept.
- Safe rendering: sanitized HTML, images off by default, clear delete action.
- Abuse Safeguards
- Rate limits on message count per mailbox/IP.
- Block forwarding to known disposable networks.
- Alert on sudden spikes from single source ASNs.
Case Study: Lab Results (2025-02)
- Test setup: 5 domains, MX on two providers, SPF/DKIM/DMARC aligned, TLS enforced.
- Delivery tests: 2000 verification emails across Gmail, Outlook, Yahoo.
- Outcome: 93–96% inbox placement (consumer), 80–85% on strict enterprise filters when domains appear in disposable lists.
- Mitigations: Domain rotation improved enterprise acceptance by ~5–8%; avoiding HTML tracking improved some corporate passes.
- Latency: P50 3.8s, P90 7.2s from SMTP accept to inbox display on our UI.
Troubleshooting Guide
- Symptom: Messages delayed >30s
- Check upstream throttling, greylisting, and queue backlogs.
- Verify DNS lookups aren’t timing out; enable resolver caching.
- Symptom: Sudden hard bounce spike
- Inspect RBL listings; rotate domains; validate SPF/DKIM keys.
- Check HELO/EHLO hostname consistency and PTR records.
- Symptom: Gmail spam folder hits increase
- Ensure DKIM alignment; avoid spammy HTML; remove tracking pixels.
- Reduce repeated identical messages; introduce slight template variation.
- Symptom: Outlook temp failures
- Honor 4xx retry windows; slow down reconnections; verify TLS handshake quality.
Automation Ideas
- Health checks: Cron to run
swaks against MX; verify headers and TLS.
- Alerting: Trigger on bounce rate >2x baseline, RBL sightings, or latency spikes.
- Domain warmup scripts: Gradually increase traffic; pause on anomalies.
- Policy as code: Store DNS templates (SPF/DKIM/DMARC) and MTA configs in Git; deploy via CI.
Compliance Considerations
- Data minimization: Never persist message bodies longer than necessary (1–24h). Keep logs short-lived (24–72h).
- User consent: If you use analytics or performance cookies in the web UI, load only after consent; provide an opt-out.
- Security baseline: CSP, HSTS, X-Content-Type-Options, Referrer-Policy, and secure cookies on any authenticated surfaces.
Deliverability Checklist
- [ ] SPF/DKIM/DMARC published, aligned, and monitored.
- [ ] MX hosts healthy, TLS enforced, PTR correct.
- [ ] Size limits, connection limits, and greylist-friendly retries.
- [ ] Sanitized rendering, images off by default, tracking blocked.
- [ ] Domain pool rotation with warmup and retirement rules.
- [ ] Metrics without payloads; alerts on bounce spikes and latency.
- [ ] Abuse controls: per-IP/per-mailbox rate limits, no relays to other disposables.
- [ ] Incident playbook for RBL hits or provider-specific blocks.
Conclusion
Deliverability is not guesswork. It’s a disciplined set of DNS configurations, protocol hygiene, abuse controls, and observability—applied with privacy in mind. With the practices above, a disposable email service can stay reliable for legitimate short-term use while minimizing exposure to spam filters, blocklists, and user risk.