Understanding Email Bounce Types and How to Fix Them
Deep dive into hard bounces, soft bounces, and block bounces. Learn what causes each type and how to prevent them.
Email bounces are failed delivery attempts, but not all bounces are created equal. Understanding the difference between hard bounces, soft bounces, and block bounces is essential for maintaining your sender reputation and improving deliverability.
Hard Bounces: Permanent Failures
A hard bounce occurs when an email cannot be delivered due to a permanent reason. These are the most critical bounces to address because they directly damage your sender reputation.
Common Causes of Hard Bounces
Invalid Email Address
The email address doesn't exist. Common causes include typos (gmial.com instead of gmail.com), fake signups, or deleted accounts.
550 5.1.1 User unknown
Domain Doesn't Exist
The domain name in the email address is not registered or has expired. This often happens with corporate domains that shut down.
550 5.1.2 Host unknown
Email Server Rejection
The recipient's mail server permanently rejects emails from your domain or IP address, often due to being blocklisted.
554 5.7.1 Service unavailable
Critical Action Required
Remove hard bounce addresses immediately and permanently from your email list. Continuing to send to hard bounce addresses severely damages your sender reputation and can lead to blocklisting.
Soft Bounces: Temporary Issues
Soft bounces are temporary delivery failures that may resolve on their own. The email address is valid, but something is preventing delivery right now.
Common Causes of Soft Bounces
Mailbox Full
The recipient's inbox has reached its storage limit. This is common with free email providers that have small mailbox quotas.
552 5.2.2 Mailbox full
Server Temporarily Unavailable
The receiving mail server is experiencing temporary issues, maintenance, or high load. Usually resolves within hours.
421 4.3.2 Service unavailable
Message Too Large
Your email exceeds the recipient server's size limit. Most servers accept up to 10-25MB, but limits vary.
552 5.2.3 Message size exceeds fixed limit
Recipient Away (Auto-Reply)
The recipient has an out-of-office auto-responder set up. The email was likely delivered, but you received an automated response.
Auto-reply notification
How to Handle Soft Bounces
Retry Strategy
Most email service providers (ESPs) automatically retry soft bounces. A typical retry schedule:
- • Retry after 15 minutes
- • Retry after 1 hour
- • Retry after 4 hours
- • Retry after 24 hours
- • After 3-5 failed retries over 72 hours, treat as hard bounce
Block Bounces: Reputation Issues
Block bounces occur when the receiving server actively rejects your email based on content, sender reputation, or policy violations. These require immediate investigation.
Types of Block Bounces
Blocklist (Blacklist)
Your IP address or domain appears on a public spam blocklist like Spamhaus, Barracuda, or SORBS. This is serious and requires immediate remediation.
554 5.7.1 Blocked - see https://blocklist.example
Content Filtering
Your email content triggered spam filters due to spammy words, excessive links, suspicious attachments, or other red flags.
550 5.7.1 Message rejected as spam
Authentication Failure
Your email failed SPF, DKIM, or DMARC checks. The receiving server doesn't trust that you're authorized to send from this domain.
550 5.7.1 SPF check failed
Policy Violation
The email violates the recipient organization's email policy, such as blocking all marketing emails or specific content types.
550 5.7.1 Policy restriction
Bounce Rate Thresholds
Understanding acceptable bounce rates helps you gauge list health:
Excellent
Below 1%
Good
1-2%
Acceptable
2-5%
Critical
Above 5%
Prevention Strategies
1. Validate Before Sending
The best way to prevent bounces is to validate email addresses before adding them to your list. Real-time validation at signup catches 90%+ of potential bounces.
2. Use Double Opt-In
Require new subscribers to click a confirmation link. This verifies the address works and that the person actually wants your emails.
3. Regular List Cleaning
Email addresses decay at about 22.5% per year. Run bulk validation quarterly to catch addresses that have become invalid since signup.
4. Monitor Authentication
Ensure your SPF, DKIM, and DMARC are properly configured and monitor DMARC reports to catch authentication issues before they cause block bounces.
5. Segment and Re-engage
Track engagement metrics. Before removing inactive subscribers, send a re-engagement campaign asking if they still want to hear from you.
Reading Bounce Error Codes
SMTP error codes follow a standard format. Here's how to interpret them:
550 5.1.1 <user@example.com>: Recipient address rejected: User unknown
550 - Primary error code (5xx = permanent failure, 4xx = temporary)
5.1.1 - Enhanced status code (category.subject.detail)
Message - Human-readable explanation
Quick Reference
- 4xx codes - Temporary failures (soft bounces)
- 5xx codes - Permanent failures (hard bounces or blocks)
- x.1.x - Addressing errors (user unknown, domain unknown)
- x.2.x - Mailbox errors (mailbox full, disabled)
- x.7.x - Security/policy errors (blocked, filtered)