Email deliverability incident response
Use this incident library when sending, delivery, placement or reliability has changed and you need a bounded diagnosis before changing a mail stream.
Start with the observable symptom and affected path. Use a specific SMTP error page when you have its exact response; use the Guide for background implementation; use these runbooks to triage an active incident.
- Authentication or DNS failsAn SPF, DKIM, DMARC or PTR result is failing or inconsistent.
- Mail is rejected, deferred or missingAn SMTP reply, rate limit or mailbox-provider outcome needs classification.
- Recipients complain or cannot unsubscribeConsent, List-Unsubscribe or complaint signals require containment.
- Sending systems are delayed or duplicatedQueues, webhooks and transactional flows need operational evidence.
Preserve the affected message, SMTP response and timeline before making a broad change.
- Spam and Inbox Placement IncidentsSpam, junk and inbox placement.9 questions
- Authentication and DNS FailuresAuthentication and DNS failures.5 questions
- Reputation, Blocklists and Warm-upReputation, blocklists and warm-up.5 questions
- Bounces, Rejections, and ThrottlingBounces, rejections and throttling.12 questions
- Complaints, Unsubscribe and Compliance IncidentsComplaints, unsubscribe and compliance.11 questions
- Transactional Email OutagesTransactional-email outages.10 questions
- Infrastructure, ESP and Migration IncidentsInfrastructure, ESP and migration incidents.8 questions
- List Quality and Suppression FailuresList quality and suppression failures.6 questions
- Monitoring and Postmaster SignalsMonitoring and postmaster signals.10 questions
- Security, Spoofing and Account CompromiseSecurity, spoofing and account compromise.8 questions
- Queues, Webhooks, and Sending ReliabilityQueues, webhooks and sending reliability.6 questions
- Mailbox Provider-specific IncidentsMailbox-provider incidents.10 questions
Related resources
Guide
- Deliverability
Inbox placement and SMTP status fundamentals.
- Sending Reliability
Retry, queues, and reliability under failure.
Error codes
- Why Was the Email Rejected?
Full SMTP enhanced-status-code catalog.
Email deliverability expert
Wojtek Blazalek
Stuck in an email incident? I help teams get delivery, reputation and auth back on track.
Hands-on deliverability work for teams that send at scale