Definition
Blocked rate measures the percentage of emails that are rejected at the server level by the recipient's mailbox provider — the email never reaches the recipient's inbox or spam folder because the receiving server refuses it entirely. This is distinct from a bounce, where the email was accepted but could not be delivered.
A high blocked rate indicates serious deliverability issues. Common causes include the sending IP or domain appearing on a blacklist, failed authentication checks, or the receiving server applying a blanket block based on sender reputation. Investigating and resolving blocks should be a top priority when they occur.
Blocked Rate vs Bounce Rate
| Metric | Definition | Implication |
|---|---|---|
| Blocked rate | Rejected by server before filtering | Sender reputation or blacklist issue |
| Hard bounce rate | Accepted but undeliverable (bad address) | List quality issue |
| Soft bounce rate | Accepted but temporarily undeliverable | Temporary issue (full inbox, etc.) |
| Complaint rate | Accepted, delivered, marked as spam | Content or targeting issue |
Best Practices
- Monitor blocked rate in your ESP dashboard after every send
- Investigate any blocked rate above 2% immediately
- Check major blacklists (Spamhaus, Barracuda, SpamCop) if blocks spike
- Verify SPF, DKIM, and DMARC authentication records
- Review sending volume patterns — sudden increases can trigger blocks
- Use a deliverability monitoring service for real-time block alerts
- Maintain relationships with ISP postmaster teams for block resolution
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Frequently Asked Questions
For a healthy email programme, blocked rate should be below 2%. Rates consistently above 5% indicate deliverability problems that require investigation. New senders or recently-warmed IPs may see higher blocked rates during the reputation-building phase.
Check your IP and domain against major blacklists first. Verify your authentication records have not changed. Review your recent sending volumes for unusual patterns. Check your complaint rate. Contact your ESP's deliverability team for guidance. Do not send additional volume until you understand the cause.