Microsoft 365 Error Codes

How to Resolve KEO EFZ4 Email Throttling Errors After Microsoft 365 Migration

WPS Editor Sep 27, 2026 869 views

Question details

The KEO application displays an EFZ4 BAD Request error due to email throttling limits being exceeded after migrating to Microsoft 365.

How to Resolve KEO EFZ4 Email Throttling Errors After Microsoft 365 Migration

Product: Microsoft 365 / KEO Application
Device & OS: not provided
Scenario: Using the KEO application to retrieve emails following a recent tenant migration to Microsoft 365.
Observed behavior: The system throws an EFZ4 BAD Request throttling error, preventing successful email retrieval because requests exceed the allowed server rate.

Before you start

Ensure you have administrative access to the KEO application settings and can modify the email synchronization configurations.

Solution 1Recommended

Adjust KEO Request Frequency and Retry Behavior

Modify your application's settings to prevent exceeding Microsoft 365's allowed rate limits.

Microsoft 365 enforces strict throttling limits to ensure service availability across all tenants. If the KEO application sends too many requests in a short timeframe, the server rejects them with an EFZ4 Bad Request error.

Check Request Frequency

Open the KEO application administration panel and navigate to the email retrieval or synchronization schedule settings.

Implement Request Batching

Configure the application to batch email requests rather than pulling them individually at high frequencies, which triggers API rate limits.

Add Delays Between Retries

Adjust the failure retry behavior to implement exponential backoff. Specifically, avoid immediate and repeated retries when an EFZ4 error is encountered.

Adjust KEO Request Frequency and Retry Behavior

Throttling Relief: Once the request rate drops below the threshold, Microsoft 365 will automatically lift the temporary throttle, and emails will resume syncing normally.

Solution 2

Seek FastTrack Community Support

Consult Microsoft's specialized community for FastTrack migrations if configuration changes do not resolve the issue.

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Frequently Asked Questions

What does the EFZ4 BAD Request error mean in Microsoft 365?

The EFZ4 error signifies that an application (such as KEO) is sending email retrieval requests at a speed or volume that exceeds Microsoft 365's permitted throttling limits.

Why did this throttling issue only appear after migrating to Microsoft 365?

Microsoft 365 implements strict resource management and API throttling policies to ensure fairness and server stability. These default cloud limits are often more restrictive than previous on-premises server configurations.

How can I prevent email throttling when using third-party apps?

You can prevent throttling by optimizing your app's request frequency, implementing request batching, and using exponential backoff (adding delays) instead of triggering immediate retries upon a failure.

Can I increase the throttling limits for my Microsoft 365 tenant?

Throttling limits in Microsoft 365 are generally fixed by Microsoft to protect overall service health. Modifying and optimizing the application's request patterns is the recommended and most effective solution.

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