What does it mean when an email bounces?
An email bounce means the recipient’s mail server received your message but refused to deliver it to the mailbox. The server sends back an SMTP error code, which your sending system records as a bounce. You may also see this referred to as a Non-Delivery Report (NDR) or Delivery Status Notification (DSN).
Hard Bounce vs Soft Bounce
The two main types of bounces:
- Hard bounce: Permanent failure. The address doesn’t exist, the domain has no mail server, or the recipient has blocked you. Common codes:
550,551,553.
Action: Suppress the address immediately — never send to it again. - Soft bounce: Temporary failure. Examples include a full mailbox, temporary server issue, or rate limiting. Common codes:
421,450,452.
Action: Retry with exponential backoff (usually up to 72 hours).
Common Reasons Emails Bounce
- Invalid or outdated email address (typos, closed accounts)
- Recipient’s mailbox is full or over quota
- Authentication failures (SPF, DKIM, or DMARC)
- Sending IP or domain is on a blocklist
- Low sender reputation due to complaints or volume spikes
- Content triggers spam or phishing filters
What to Do When Emails Bounce
- Always check the full bounce reason and SMTP code.
- Immediately suppress hard bounces.
- Retry soft bounces using a sensible backoff schedule.
- Monitor your overall bounce rate — healthy lists stay under 2%.
For a detailed remediation playbook, bounce code reference, and automation tips, see the Email Bounces Guide.