v2.9 engineering library
SMTP Operations & Incident Runbooks
Use exact remote responses and queue evidence to choose the right operational action.
SMTP 421 Deferrals: Diagnosis and Recovery
Separate temporary provider pressure from local capacity, DNS, connection and reputation problems.
SMTP 450 Mailbox Unavailable
Interpret temporary mailbox failures without misclassifying them as permanent bounces.
SMTP 451 Local Processing Errors
Troubleshoot temporary remote processing failures and distinguish them from sender-side faults.
SMTP 452 Storage and Recipient Limits
Handle temporary storage and recipient-limit responses safely.
SMTP 550 Rejections
Classify permanent rejection reasons before suppressing recipients or changing infrastructure.
SMTP 552 Message Size and Storage Rejections
Resolve permanent message-size and storage-related failures.
SMTP 554 Transaction Failed
Investigate broad transaction failures involving policy, reputation, content or protocol.
Queue Backlog Recovery Runbook
Recover from a growing outbound queue without causing a second incident.
Retry Policy Engineering
Design retry schedules that respect provider signals and operational objectives.
SMTP Concurrency Tuning
Tune simultaneous connections using acceptance, latency and deferral evidence.
SMTP TLS Handshake Failures
Diagnose certificate, protocol, cipher and interception problems.
MX and DNS Resolution Failures
Separate authoritative DNS errors, resolver failures and incorrect routing.
Microsoft S775 and Reputation Throttling
Respond to Microsoft reputation blocks with controlled traffic and documented remediation.
Gmail 421 4.7.0 Deferrals
Troubleshoot Gmail temporary rate and reputation deferrals.
Yahoo TS03 and TS04 Deferrals
Handle Yahoo/AOL temporary deferrals caused by traffic or reputation pressure.
Unauthorized MAIL FROM Incident Response
Contain SMTP credential abuse and prevent unapproved envelope senders.