central_filter defer (-17): error in filter file: unknown

Knowledgebase

Errors & Troubleshooting | Updated 2026

The "central_filter defer (-1): error" message in email bounce notifications indicates that the server's email filtering system encountered an issue processing the message. This is usually a temporary server-side condition.

What to Do

Wait and retry, or contact support

This error is typically transient. Wait 15-30 minutes and try sending again. If the error persists, open a support ticket with the full bounce message so we can check the server's mail filter configuration.

01. Common Causes

SpamAssassin or email filter overload - The filtering process timed out while scanning the message
Large attachment - Very large emails take longer to scan and may exceed filter timeouts
Temporary server issue - The filtering service may have been restarting or under heavy load
Malformed email headers - Unusual or broken headers can cause the filter to choke

02. Workarounds

Reduce attachment size - Use a file sharing service instead of large email attachments
Try sending without attachments to isolate the issue
Send from webmail to test if the issue is with your email client

For general email troubleshooting, see our Email Troubleshooting Guide.