What Is an Email Bounce?
An email bounce occurs when a message cannot be delivered to the intended recipient and is returned to the sender. The bounce notification, formally called a Delivery Status Notification (DSN) or Non-Delivery Report (NDR), is generated by the receiving mail server or an intermediate relay and sent back to the envelope sender (return-path) address.
Hard Bounces vs. Soft Bounces
Hard Bounces
A hard bounce is a permanent delivery failure. The message will never be deliverable to that address. Common causes:
- Invalid recipient. The email address does not exist (e.g., typo in the address, account deleted).
- Invalid domain. The domain has no MX records or does not exist in DNS.
- Permanent rejection. The receiving server explicitly refuses the message with a 5xx status code.
Hard bounces should trigger immediate removal of the address from your mailing list. Continuing to send to hard-bounced addresses damages your sender reputation.
Soft Bounces
A soft bounce is a temporary delivery failure. The message might be deliverable if retried later. Common causes:
- Mailbox full. The recipient's storage quota is exceeded.
- Server temporarily unavailable. The receiving server is down or overloaded.
- Message too large. The message exceeds the recipient server's size limit.
- Temporary policy rejection. Rate limiting, greylisting, or temporary blocklisting.
SMTP servers typically retry soft-bounced messages over a period of hours or days (configurable in the MTA) before converting them to permanent failures.
Common SMTP Bounce Codes
SMTP uses three-digit reply codes (RFC 5321) and enhanced status codes (RFC 3463) to communicate delivery results:
| Code | Enhanced | Meaning |
|---|---|---|
| 550 | 5.1.1 | User unknown / mailbox does not exist |
| 550 | 5.7.1 | Message rejected by policy (e.g., failed authentication) |
| 551 | 5.1.6 | Recipient has moved |
| 552 | 5.2.2 | Mailbox full |
| 553 | 5.1.3 | Invalid address syntax |
| 421 | 4.7.0 | Temporary rejection, try again later |
| 450 | 4.2.1 | Mailbox temporarily unavailable |
| 451 | 4.7.1 | Greylisting (intentional temporary rejection for new senders) |
The first digit indicates the category: 5xx codes are permanent failures (hard bounces), and 4xx codes are temporary failures (soft bounces).
Impact on Sender Reputation
Bounce rates directly affect your domain and IP reputation at major mailbox providers:
- Gmail monitors bounce rates through Postmaster Tools. A bounce rate above 2% is a warning sign; above 5% triggers filtering.
- Outlook/Microsoft evaluates bounce patterns as part of its Smart Network Data Services (SNDS) scoring.
- Yahoo factors bounce rates into sender reputation alongside complaint rates and engagement signals.
High bounce rates signal to providers that you are not maintaining a clean list, which correlates with spammy sending behavior. This can lead to increased spam folder placement or outright blocking.
Reducing Bounces
- Implement double opt-in to verify email addresses at signup.
- Remove hard bounces immediately after the first occurrence.
- Remove soft bounces after repeated failures (typically 3-5 consecutive bounces).
- Validate email addresses at the point of collection using syntax checks and, where possible, SMTP verification.
- Monitor your MX records and sending infrastructure with a full domain analysis to ensure your own receiving and sending setup is healthy.
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