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SMTP Bounce Code Lookup

Look up any SMTP bounce code or enhanced status code and get a plain English explanation with actionable fixes. Covers all standard SMTP reply codes from 2xx to 5xx.

About this tool

When an email bounces, your mail server receives an SMTP reply code that explains why delivery failed. These codes range from simple three-digit numbers like 550 to enhanced status codes like 5.1.1. Understanding what each code means is the first step to fixing deliverability problems and keeping your sender reputation healthy.

This lookup tool covers all standard SMTP response codes defined in RFC 5321 and enhanced status codes from RFC 3463. Search by code number, enhanced code, or keywords like "mailbox," "authentication," or "quota" to quickly find the information you need.

Hard bounces vs. soft bounces

Hard bounces (5xx codes) are permanent failures. The most common is 550, meaning the mailbox does not exist. You should remove hard-bounced addresses from your list immediately since continued sending damages your domain reputation. Soft bounces (4xx codes) are temporary failures like full mailboxes or server outages. Most email systems retry soft bounces automatically, but addresses that soft bounce repeatedly over days should be suppressed.

Common bounce scenarios

The most frequent bounce code is 550 5.1.1 (mailbox not found), which accounts for the majority of hard bounces. Use our email validator to catch invalid addresses before sending. Code 451 (temporary server error) often indicates greylisting, where the server intentionally rejects the first delivery attempt and accepts retries. Code 554 5.7.1 usually means authentication failures, so check your SPF, DKIM, and DMARC records.

High bounce rates above 2% can trigger spam filtering and account suspensions. Monitor your bounce rate after every campaign and investigate unusual spikes. Our blacklist checker can help identify if bounce-related reputation damage has led to blacklisting.

For email marketers managing large lists, regular list hygiene is essential. Validate addresses before importing, remove hard bounces immediately, suppress chronic soft bouncers, and use double opt-in to prevent invalid signups. These practices keep your deliverability score high.

Frequently Asked Questions

What is the difference between a hard bounce and a soft bounce?

A hard bounce (5xx SMTP code) is a permanent delivery failure. The email address does not exist, the domain is invalid, or the server permanently refuses your email. Remove hard-bounced addresses immediately. A soft bounce (4xx code) is a temporary failure like a full mailbox, server outage, or message too large. Soft bounces are retried automatically and may succeed later.

What does bounce code 550 mean?

Code 550 means 'Mailbox not found' or 'Access denied.' It is the most common hard bounce code. In most cases, it means the email address does not exist. The enhanced code 550 5.1.1 specifically confirms a non-existent mailbox. Remove these addresses from your list immediately to protect your sender reputation.

What is an enhanced status code?

Enhanced status codes (defined in RFC 3463) provide more detail than basic 3-digit SMTP codes. They follow the format X.Y.Z where X is the class (2=success, 4=temp failure, 5=permanent failure), Y is the subject (1=address, 2=mailbox, 3=system, 4=network, 5=protocol, 7=security), and Z provides specific detail. For example, 5.1.1 means permanent address failure, specifically bad mailbox.

What bounce rate is acceptable?

Industry best practice is to keep your bounce rate below 2%. Email service providers like Gmail and Yahoo closely monitor bounce rates. Rates above 5% can trigger spam filtering, and rates above 10% may result in account suspension. Hard bounces should be near 0% if you validate addresses before sending.

Why am I getting 451 errors (greylisting)?

Greylisting is an anti-spam technique where a mail server temporarily rejects the first delivery attempt from an unknown sender. Legitimate mail servers will retry delivery after a delay (usually 5-15 minutes), while spammers typically do not retry. If you see 451 errors, your email system should handle retries automatically. The message will be accepted on the second or third attempt.

What does error 554 5.7.1 mean?

Error 554 5.7.1 means 'Delivery not authorized' and usually indicates an authentication or policy rejection. Common causes include failed SPF authentication (your sending IP is not authorized), failed DKIM signature verification, DMARC alignment failure, or your IP/domain being on a blocklist. Check your email authentication records and sender reputation.

Should I retry soft bounces?

Yes, soft bounces should be retried automatically by your email system. Most email service providers retry for 24-72 hours before converting to a permanent failure. If an address soft bounces consistently across multiple campaigns (3+ times in 30 days), consider suppressing it since the underlying issue may be persistent.

How do I reduce my bounce rate?

Use double opt-in for new subscribers to verify email addresses are real. Validate email addresses before importing lists using an email validation tool. Remove hard bounces immediately after each send. Clean your list every 3-6 months by removing addresses that have not engaged. Authenticate your domain with SPF, DKIM, and DMARC to avoid policy rejections.

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