Troubleshooting Playbooks
Evidence-first triage, decision paths, recovery and prevention for common mail-infrastructure failures.
SMTP AUTH failures
Authentication errors, credential scope, TLS requirements and application configuration.
Unauthorized MAIL FROM use
Authenticated users submitting unapproved envelope sender domains.
SMTP listener connection refused
Service, bind address, firewall and port-conflict diagnosis.
Relay denied
Source authorization, authentication and recipient relay policy.
Rapid queue growth
Separate intake pressure, provider slowdown, DNS faults and local resource constraints.
Oldest message age rising
Identify whether delivery capacity or retry policy is preventing queue recovery.
Queue disk pressure
Contain intake, preserve critical mail and restore storage headroom.
DNS timeouts
Resolver health, reachability, cache behavior and source-IP routing.
PTR and HELO mismatch
Correct forward and reverse identity without introducing DNS inconsistency.
DKIM verification failure
Selector publication, key mismatch, canonicalization and body modification.
SPF failure
Envelope sender domain, source IP authorization and lookup-chain problems.
DMARC failure
Identifier alignment, SPF/DKIM pass results and From-domain policy.
TLS handshake failure
Certificate chain, hostname, protocol compatibility and time validity.
Microsoft S775 throttling
Evidence collection, rate stabilization and reputation recovery controls.
Microsoft 421 deferrals
Differentiate reputation throttling, resource pressure and transient provider conditions.
Gmail 421 temporary failures
Review reputation, recipient quality, authentication and traffic changes.
Gmail spam placement
Separate delivery acceptance from inbox placement and inspect engagement/list quality.
Yahoo/AOL 421 deferrals
Review provider responses, complaints, reputation and connection policy.
GMX/WEB.DE deferrals
Conservative rate adjustments and identity verification.
Apple iCloud deferrals
Inspect temporary responses, complaint signals and sender identity.
High hard-bounce rate
Pause affected sources, verify acquisition and suppress invalid recipients.
Complaint spike
Contain the campaign, map complaints to tenant/source and suppress immediately.
Bounce processing gap
Verify Return-Path, inbound DNS, parser and tenant mapping.
Accounting log gap
Check file paths, permissions, rotation, disk and ingestion checkpoints.
Wrong source IP used
Inspect VMTA assignment, pools, pattern lists and default routing.
Wrong HELO used
Trace VMTA selection and smtp-source-host identity.
MailWizz submission failure
Credentials, port, TLS mode, listener and application logs.
Postfix relay failure
relayhost syntax, transport maps, SASL and local network reachability.
PowerMTA startup error
Locate line-specific configuration faults and unsupported syntax.
Unexpected queue removal
Audit bounce policy, manual commands, scripts and filesystem events.