400 | Bad Request
The server could not understand the request due to invalid syntax or malformed data.
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Struggling with HTTP 404s, email bounce-backs, or WordPress white screens? You're in the right place. This error reference guide breaks down the most common HTTP, email, and WordPress errors, explaining what they mean, why they happen, and how to fix them in plain, actionable language. Whether you're a developer, sysadmin, or business owner, this guide gives you a clear direction for every error.
A reference for the most common HTTP status codes: what they mean, what causes them, and how to fix them.
The server could not understand the request due to invalid syntax or malformed data.
Authentication is required or the provided credentials are invalid.
The server understood the request but refuses to authorize it.
The requested resource or page could not be found on the server.
The HTTP method used is not permitted for this endpoint.
The server timed out waiting for the client to send the request.
The uploaded file or request body exceeds the server's size limits.
The client has sent too many requests in a given amount of time (rate limiting).
A generic server-side error occurred; the server encountered an unexpected condition.
A server acting as a gateway received an invalid response from an upstream server.
The server is temporarily overloaded or down for maintenance.
A gateway or proxy server did not receive a timely response from an upstream server.
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A reference for the most common SMTP email error codes: what they mean, what causes them, and how to fix them.
The mail server is temporarily unavailable or overloaded and cannot process the request right now.
The requested mailbox is temporarily inaccessible, possibly due to a lock or system issue.
A temporary server-side processing failure occurred, often due to anti-spam checks or greylisting.
The mailbox or server storage limit has been reached and cannot accept new messages.
The server could not recognize the command due to a syntax error in the SMTP conversation.
The mail server is temporarily unable to process the request, often due to high load or maintenance.
SMTP authentication was rejected due to incorrect username, password, or auth method.
The recipient email address does not exist on the server or was permanently rejected.
The recipient is not hosted on this mail server and the server will not forward the message.
The email exceeds the maximum allowed message size set by the receiving server.
The recipient address contains invalid characters or formatting the server cannot accept.
The message was rejected due to spam filtering, policy rules, or a blacklisted sending IP.
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A reference for the most common WordPress errors: what they mean, what causes them, and how to fix them.
WordPress cannot connect to the MySQL database, preventing the site from loading at all.
A blank white page caused by a fatal PHP error, memory exhaustion, or a plugin/theme conflict.
A generic server-side failure in WordPress, often triggered by a corrupted .htaccess file or plugin conflict.
The requested page or post does not exist, often caused by broken permalinks or missing rewrite rules.
PHP has run out of allocated memory, usually caused by resource-heavy plugins or themes.
The user is repeatedly redirected back to the login page after entering valid credentials.
The site remains in maintenance mode after an update failed or was interrupted mid-process.
Pages and posts return 404 errors due to broken or missing mod_rewrite rules in .htaccess.
The site loads over HTTPS but some resources are still served over HTTP, triggering browser warnings.
The browser is caught in an infinite redirect loop, often caused by conflicting SSL or URL settings.
Two or more plugins conflict with each other or with the active theme, causing errors or broken functionality.
WordPress cannot write to the uploads directory due to incorrect permissions on the server.
