SMTP Code: "451 4.4.1 [internal] No valid hosts (too many connection failures)"

Products

• Messaging Gateway

Issue/Introduction

Messages are queued in the delivery queue with a "451 4.4.1 [internal] No valid hosts (too many connection failures)"

Cause

Repeated failures to connect to an SMTP host have resulted in the entire domain being marked as failed, and consequently, all delivery attempts have been temporarily suspended.

Resolution

The exact conditions under which this occurs and how to resolve it are being researched.

Conditions under which this has been seen:

1. Scanner unable to deliver spam to quarantine due to improper MTA binding. This can be addressed by binding "Messages destined for the Control Center" to another IP. In the Control Center go to (Administration → Configuration → Select a host → SMTP → Advanced Settings → Delivery).
2. Outbound delivery to specific hosts or to all hosts fails due to firewall configuration preventing establishment of an SMTP connection.
   This issue can be addressed by opening up and/or reconfiguring the firewall not to filter SMTP traffic.
3. Check email delivery to a specific external domain that is in the delivery queue failing to delivery by following the steps for outbound mail in the article Troubleshoot email delivery issues using telnet