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SMTP Response Codes
Facebook provides Enhanced Mail System Status Codes as described in RFC 1893. Facebook also provides custom error codes to help system administrators troubleshoot delivery issues.
Sender Domain Issues
These codes indicate an issue with either your domain's DNS record or your mail server's reverse DNS record.
Permanent Issues
These will persist until action is taken to correct them.

Temporary Issues
These may only persist for a short time. Administrators may want to investigate them further if they occur often.

Recipient Issues
These codes indicate an issue with one of the message recipients. Administrators observing these issues are not required to take any action to correct them.

Message Issues
These codes indicate an issue with the specific message that triggered them. Message authors observing these issues are required to take action to correct them.

Connection Limits
These codes indicate an issue with the connections from your mail servers. They are temporary conditions and administrators observing them are not required to take any action to correct them.

Policy Violations
These codes indicate that either the message or your mail server has violated Facebook's abuse prevention policies.
Permanent Mail Server Policies
These will persist until the administrator takes action to correct them.

Permanent Message Policies
Message authors observing these issues are required to take action to correct them.

Permanent Domain Policies
These will persist until action is taken to correct them.

Temporary Mail Server Policies
These will expire after the stated duration. Administrators are encouraged to take corrective action to prevent them from reoccurring.

Facebook System Resource Issues
These codes indicate a temporary issue internal to Facebook's system. Administrators observing these issues are not required to take any action to correct them.