outlook.office365.com/ews/exchange.asmx return (503) Server Unavailable

Hello @Anonymous ,

Thanks for the information.

I was able to resolve the issue by changing the URL from /ews/exchange.asmx to /EWS/Exchange.asmx.

I'm not sure what the root cause was. One possibility is that the lowercase Microsoft URL was no longer being handled correctly, even though URLs for this endpoint should generally be case-insensitive. However, I can't confirm whether that was the actual cause.

Hello @Thu Le

Firstly, I would like to clarify that this is a user‑to‑user support forum. Moderators like me are not Microsoft support, and we do not have backend access or the ability to directly intervene in Microsoft services. We can only provide technical guidance and best‑practice recommendations based on reported behavior.

To begin, it would be helpful to confirm whether you are using Exchange Online or Exchange Server. If it is Exchange Server, could you also share which version you are running? However, based on the endpoint you provided, this typically indicates that you are using Exchange Online. From what I have seen, this type of 503 error is more commonly reported in Exchange Online scenarios rather than with Exchange Server.

To help narrow things down, you may want to go through a few quick checks below, since your connector is using the Exchange Online (EWS) endpoint:

1> Try an external connectivity test: Go to the Test Exchange connectivity tool https://testconnectivity.microsoft.com/tests/exo, select “Service Account Access (Developers)”, and use OAuth with the same configuration as your application

2> Check that your token is still valid (not expired), and confirm the required permissions or scope for EWS access (for example, full_access_as_app or equivalent). You may also run a quick test using PowerShell or an EWS test script if available.

It may also help to implement retry logic (such as exponential backoff) and avoid sending large bursts of requests, which can sometimes trigger throttling.

3> Check whether the issue is affecting more than one mailbox or user. You can also review the Service Health section in the Microsoft 365 Admin Center to see if there are any ongoing issues impacting your tenant.

During my research, I have seen other users reporting similar behavior. If this issue is occurring in Exchange Online, it could be related to a service-side condition so please consider reporting the issue through the Admin Center so the support team can investigate further on the service side: https://learn.microsoft.com/en-us/microsoft-365/admin/get-help-support?view=o365-worldwide#online-support

I hope this information is helpful. I look forward to your update.

Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.