SMTP Error Code & Bounce Code Lookup
Diagnose email bounces, deferrals, and sending failures. Search any 3-digit or enhanced RFC 3463 SMTP code to understand why the recipient server rejected your email and how to resolve it.
Relay Access Denied / Transaction Failed
The receiving mail server refuses to relay mail for untrusted or unauthenticated users, or the recipient domain is not served by this SMTP server.
4xx vs 5xx SMTP Error Codes: What's the Difference?
SMTP response codes starting with 4xx represent temporary deferrals (e.g. rate limits, greylisting, server overload). The sending MTA should automatically hold the email in queue and retry later. Codes starting with5xx represent permanent rejections (e.g. user unknown, relay access denied, spam block). Messages that receive 5xx codes must not be automatically retried without configuration changes.
What are Enhanced Status Codes (e.g., 5.7.1)?
Defined in RFC 3463, enhanced status codes provide detailed sub-classifications for standard 3-digit codes. The second number indicates the category (e.g., .1 is addressing, .2 is mailbox, .7 is security/authentication), making it easier to distinguish authentication failures from address typos.
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