What it is
SMTP clients queue messages after transient failures and attempt redelivery. Application semantics may impose a shorter useful lifetime than SMTP’s general queue policy.
Why it matters
Immediate repeated retries amplify outages. Delivering an old login code, price alert, or cancellation notice can confuse or endanger recipients.
How to implement
Use increasing retry intervals with jitter, honor server guidance, cap total attempts, and attach an application expiry to time-sensitive mail. Keep idempotency across retries.
Common mistakes
- Retrying permanent 5xx responses.
- Generating a new logical message for every transport attempt.
- Applying one queue lifetime to receipts and verification codes.
Verification
Simulate sustained 4xx responses and recovery. Observe spaced attempts, stable message identity, correct final failure, and expiry before stale delivery.