Symptom: bounce events or SMTP replies with 550 mailbox unavailable, 551, or 553.
1. Treat these as hard bounces. The address does not exist or will never accept mail. Retrying is pointless and hurts reputation.
2. Remove the address from the mailing list. SendGrid's own guidance for all three codes is the same: take them off the list.
3. If you keep mailing a bounced address, SendGrid suppresses it for you (the bounce suppression list) and logs a repeat-bounce drop. Clean your list before SendGrid has to.
4. Distinguish from 421/450/451 deferrals, which are temporary and retried for up to 72 hours. 5xx is permanent, 4xx is temporary.
5. A sudden burst of 550s on previously good addresses means a typo in the recipient source, a bad import, or a compromised signup form, not a SendGrid outage. Check the source.
Wire the Event Webhook bounce events into your list management so hard bounces unsubscribe automatically.