Can ConversationID change for existing item in conversation?

Question

We are writing Outlook VSTO add-in and save info about mail items in database. Some features are relying on conversations in Outlook, so we save ConversationID in the database to detect that all messages are from the same thread.

Recently I found that at some point conversationID is changed for the entire conversation! So I have first message with ID 5E2C809C8D7E4BEBA402613311DD8C7B And then suddenly it becomes 347B0DEB35C68E1EE882EFABE4BCC373 for the next messages. And when I select previous messages they also have this 347 id. It seems that it broke when I answered an e-mail with changed subject (and it gets out of thread) or after I answered from Win10 mail client. But I can't reproduce this anymore.

Does anyone know in which cases this can happen?

Answer 1

Conversation index can change if the "Thread-Index" MIME header is missing from a message in the conversation. But it will not change for the existing older messages in a conversation. For that to happen, each and every one of them would need to be updated, and if that happened, you should see the updated last modified date.