SRC365 / Outlook Add-In: Error 500 The given header was not found

Created by Abderrahim Ibnou el kadi, Modified on Thu, 20 Mar at 11:23 AM by Marc Chouinard

Problem:

Users report error when they try to report/block a message using the Outlook Add-In.

Error sample:

Failed to report this email as spam (Reason: 500 The given header was not found)

Solution:

The issue is that the plugin uses Exchange Tokens that will be deprecated in 2025. Microsoft has already turned on deprecation but in the meantime, you can override the deprecation by enabling the legacy Exchange Tokens via a powershell script.

You need to do this with Global Admin credentials for the entire O365 tenant. This needs to be done *before* June 2025. Script can be run on any machine.

WINDOWS 10

Open the PowerShell (admin mode) prompt and run the following commands:

Install-Module -Name ExchangeOnlineManagement
Update-Module -Name ExchangeOnlineManagement
Connect-ExchangeOnline
Set-AuthenticationPolicy –AllowLegacyExchangeTokens -Identity "LegacyExchangeTokens"

WINDOWS 11 / SERVER 2022

Still in QA - Will update the KB once confirmed

It can takes up to 24 hours for Microsoft to proceed with the re-activation.