Graph API /me/messages/{id}/reply returns UnknownError intermittently

Question

We are calling the Microsoft Graph API to reply to an existing message using:

POST /v1.0/me/messages/{OldMessage.id}/reply

Occasionally, the call fails with the following response:

{
  "error": {
    "code": "UnknownError",
    "message": "",
    "innerError": {
      "date": "2025-08-13T12:06:31",
      "request-id": "ef2e8d46-555c-4071-8e0c-10910ccbb508",
      "client-request-id": "ef2e8d46-555c-4071-8e0c-10910ccbb508"
    }
  }
}

---

Expected Behavior

The API should successfully send the reply without returning UnknownError.

---

Actual Behavior

The same message ID sometimes works and sometimes returns UnknownError.

The error occurs intermittently, not for all requests.

• The message property in the error is empty, making troubleshooting difficult.

Questions for Microsoft

What are common causes for UnknownError in /reply calls?

  Is this error related to mailbox state (moved/deleted/draft changes) at the time of the call?
  
     Can you trace this `request-id` to identify the root cause?
     
        Are there known reliability issues or required retry strategies for the `/reply` endpoint?
        

Answer 1

Hello Dev,

I understand you're experiencing intermittent UnknownError responses with the Microsoft Graph API /me/messages/{id}/reply endpoint. This is genuinely one of the most frustrating types of issues to deal with since it works inconsistently with the same message ID.

What's Actually Happening

Looking at your error response, the empty message field is unfortunately typical of Microsoft Graph's less helpful error messages. The intermittent nature - where the same message ID sometimes works and sometimes doesn't - is a clear indicator that this is a service reliability issue on Microsoft's end rather than a problem with your code.

Common Causes You Questioned

Several factors commonly contribute to this specific error pattern:

Service synchronization delays: Exchange Online operates on a distributed architecture where different backend services need to stay synchronized. When you attempt to reply to a message, the service handling your request may not have the most current state of that message, leading to the failure.

Backend infrastructure issues: Microsoft's Graph API services occasionally experience transient failures that aren't properly communicated through their error responses. The fact that retrying the same request often succeeds points directly to this.

Concurrent access conflicts: If the original message is being accessed or modified by other clients (Outlook apps, mobile devices, other API calls) at the same time as your reply attempt, temporary locking or state conflicts can occur.

Mailbox state transitions: Messages that have recently been moved, had permissions changed, or are undergoing other mailbox operations can temporarily be in states that prevent reply operations.

Recommended Solutions

Implement exponential backoff retry logic: This isn't optional when working with Microsoft Graph - it's essential. Start with a 1-second delay, then 2, 4, 8 seconds for subsequent retries. Most of these errors resolve within 2-3 retry attempts.

Add message validation before replying: Make a quick GET request to /me/messages/{id} before attempting the reply to ensure the message exists and is accessible.

Comprehensive logging and monitoring: Track all request IDs (like your ef2e8d46-555c-4071-8e0c-10910ccbb508) along with timestamps and error patterns. This data becomes invaluable for both your own debugging and potential Microsoft Support cases.

Consider alternative approaches: For critical workflows, you might implement a queue-based system where failed replies are retried after a longer delay, or explore using Microsoft Graph's batch operations where applicable.

Microsoft Support Escalation

Given the service-side nature of this issue, opening a Microsoft Support case with your request ID and error details would be valuable. Reference the specific endpoint /me/messages/{id}/reply and emphasize the intermittent nature of the failures. Microsoft can trace the backend behavior using your request ID to identify root causes on their infrastructure.

This type of reliability issue with Microsoft Graph messaging endpoints is unfortunately common, and while the workarounds above will improve your success rate, the underlying problem requires attention from Microsoft's engineering teams.

Hope this helps!