I have EWS code (not managed API) that creates messages in Microsoft 365 folders using “CreateItem”. In my XML I have various extended properties, plus the “MimeContent” property that contains the standard internet headers and message content. This works fine.

I would now like to group messages together so that they show up in M365 as conversations. I tried setting the internet header “Thread-Id” to the same value for a group of messages, but this had no effect. Is there a way I can do this using internet headers? Or is there an extended property I can set? I have read the Microsoft documentation about conversations but it did not seem helpful, as the relevant properties are read-only.

Or is this possible to do in Graph?

EDIT: Here is how I tried to set PR_CONVERSATION_TOPIC in my inbound XML:

<t:ExtendedProperty>
<t:ExtendedFieldURI PropertyTag="0x0070" PropertyType="String"/>
<t:Value>This is a dummy subject</t:Value>
</t:ExtendedProperty>