SendGrid hard bounce vs soft bounce (deferred): read the event, not the vibe

TL;DR

Delayed or asynchronous bounces may arrive without a message ID; match them by email address and timestamp instead. Hard bounce: remove the address from the list.

Steps

1. Symptom: delivery failures in the Event Webhook data.
2. Cause, read from the event:
3. event: bounce with a 5xx status and a bounce_classification: permanent. The mailbox is gone or will never accept mail.
4. event: deferred with a 4xx response and an attempt counter: temporary. The receiving server throttled or had a transient problem. SendGrid retries these for up to 72 hours.
5. Delayed or asynchronous bounces may arrive without a message ID; match them by email address and timestamp instead.
6. Confirmation: check the status field. 5.x.x is permanent, 4.x.x is temporary. The reason and response fields carry the server's own words.
7. Fix:
8. Hard bounce: remove the address from the list. It will land on the bounce suppression list if you keep trying.
9. Deferred: do nothing except watch. If deferrals cluster on one domain, reduce volume to that domain; more mail makes throttling worse.
10. Never 'fix' a hard bounce by resending. Never 'fix' a deferral by switching to a new IP mid-stream.
11. Verification: hard-bounced addresses stop appearing after list cleanup; deferred messages resolve into delivered events within the retry window.

When to use

You are seeing this: Symptom: delivery failures in the Event Webhook data. Use this skill when you run into "SendGrid hard bounce vs soft bounce (deferred): read the event, not the vibe".

When not to use

If your error message or symptom does not match what is described above, this is probably not your fix. Search for your exact error text instead of forcing this one to fit.

Versions

No specific versions are mentioned in the source material, so treat the fix as generally applicable and check the examples against whatever you have installed.

Why this happens

The original report does not dig into a root cause. It documents the symptom and the fix that resolved it.

The event reference: bounce events carry bounceclassification, reason, status, and response; deferred events carry attempt counts and mean the receiving server rejected the message temporarily. The SMTP errors doc: 4xx is temporary and retried up to 72 hours, 5xx is permanent and not retried. Sources: https://docs.sendgrid.com/for-developers/tracking-events/event | http://sendgrid.com/docs/Glossary/emailerror_messages.html