Hi, The following MS Graph APIs are working only for primary mailbox and not working for shared mailbox. Hence there is no way to set/get custom properties for emails in the shared mailbox. This is required for classifying emails as processed based on these custom properties. " POST https://graph.microsoft.com/v1.0/users/<sharedmailbox ID>/extensions Content-type: application/json{ "@odata.type":"microsoft.graph.openTypeExtension", "extensionName":"<extensionName>", "extensionProperty": <extensionProperty> } GET `https://graph.microsoft.com/v1.0/users/<sharedmailbox ID>/messages/${itemId}/extensions/Microsoft.OutlookServices.OpenTypeExtension.iM.docId`; PATCH `https://graph.microsoft.com/v1.0/users/<sharedmailbox ID>/messages/${itemId}/extensions/Microsoft.OutlookServices.OpenTypeExtension.iM.docId`; { "@odata.type":"microsoft.graph.openTypeExtension", "extensionName":"<extensionName>", "extensionProperty": <extensionProperty> }, DELETE `https://graph.microsoft.com/v1.0/users/<sharedmailbox ID>/messages/${itemId}/extensions/Microsoft.OutlookServices.OpenTypeExtension.iM.docId`; { "@odata.type":"microsoft.graph.openTypeExtension", "extensionName":"<extensionName>", }" So open extensions API (GET/POST/PATCH/DELETE) should support both primary mailbox and shared mailbox. Thanks Maju
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