Error Codes

This guide documents the error codes that ImprovMX's SMTP server returns when email sending attempts fail. Understanding these codes helps identify whether delivery issues stem from the service provider or your configuration.

Error Codes Reference

Code	Description
5.9.0	Loop Detection - Email is sent and forwarded from/to improvmx.com. We block these messages to avoid loops.
5.9.1	Bounce Prevention - Bounce messages originating from ImprovMX are blocked to prevent cascading failures.
5.9.2	Domain Not Found - The recipient's domain could not be located during processing.
5.9.3	Database Error - Our database returned invalid or corrupted data about the requested recipient's domain.
5.9.4	Anti-Spam Failure - The message failed our anti-spam security check. The sender should retry transmission.
5.9.5	Forwarding Configuration Issue - Unable to correctly read the forwarding recipient (where to forward the email to).

Error Categories

These error codes fall into three main categories:

• Loop Detection (5.9.0, 5.9.1) - Prevents infinite email loops and bounce cascades
• Anti-Spam Mechanisms (5.9.4) - Security screening failures that may be temporary
• Recipient/Domain Configuration (5.9.2, 5.9.3, 5.9.5) - Issues with domain lookup or forwarding settings

Note: All 5.x.x codes indicate permanent failures. If you encounter a 5.9.4 error, the sender may retry, but other codes typically indicate configuration issues that need to be addressed.