Overview

If you are relaying emails through a relay server, you might experience an issue with emails getting stuck in the message queue in your Kerio Connect system, affecting many users and persisting despite following recommended troubleshooting procedures and updating to the latest version of Kerio Connect.

A consistent error message you may see is '4.3.2 451 4.7.500 server busy'. 

This is a sample mail.log entry related to the issue:

[06/Feb/2024 11:43:38] Sent: Queue-ID: 65c20d59-000043f7, Recipient: <user@domain.com>, Result: delayed, Status: 4.3.2 451 4.7.500 Server busy. Please try again later from [XX.XX.XX.XX]. (S77719) [XXXX.eurprd03.prod.outlook.com 2024-02-06T10:43:41.195Z 08DC21D86890B5C8], Remote-Host: XXXX-com.mail.protection.outlook.com, Msg-Id: <XXX@domain.com>

Solution

This issue is usually related to relay server SMTP configurations, and needs to be checked with the relay service provider.

To help isolate the issue, you can send messages to the relay server directly through any SMTP client, and check if the email is delivered or getting errors.

Summary

In cases where emails are stuck in the message queue in Kerio Connect and the error message '4.3.2 451 4.7.500 server busy' is consistently appearing, the issue can be resolved by relay server providers making adjustments on their end. 

FAQ

Q: What does the error message '4.3.2 451 4.7.500 server busy' mean?
A: This error message typically indicates that the server is too busy to process the request at the moment.