M365 Failing to Send Emails - Timeout Calling MS365

Last Updated: Sep 10, 2025

Overview

This article explains a potential reason for not receiving any email notifications. The SMTP provider mentions a delivered status, but the log mentions timeout on MS365.

Timeout calling MS365

Applies To

Microsoft 365

Cause

Rate limits are being reached on the M365 SMTP server.

Solution

The "MdbReplicationProtectionUtilizationMonitor is unhealthy" error message refers to throttling issue with error code 503.

When it comes to MS Graph API throttling:

Reduce the number of operations per request.
Reduce the frequency of calls.
Avoid immediate retries, because all requests accrue against your usage limits.

There are some best practices to avoid the throttling while using the Graph API requests.

Wait the number of seconds specified in the retry-after header.
Retry the request.
If the request fails again with a 429-error code, throttling is still taking place. Continue to use the recommended retry-after delay and retry the request until it succeeds.