SMTP Bounce Code Lookup

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.