Message id is different for mail which is in Inbox & Sent items

Source: Microsoft Support

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If we switch to Online mode after sending the message in outlook Cached mode, the message in your Sent Items in the mailbox (not the OST) contains a different value for the PR_INTERNET_MESSAGE_ID property when compared with MFCMapi.

This is expected behavior and as a workaround we could use Outlook in Online Mode.

Here is a more detailed explanation of what is happening in these 4 scenarios:

1. In Online mode, when you send a message, it is submitted to Exchange and then stamped with PR_INTERNET_MESSAGE_ID. The copy in your Sent Items and the received item in your Inbox contain the same PR_INTERNET_MESSAGE_ID because they originated from the same message.
2. If you then switch to Cached mode after sending the message in Online mode, the message from your Sent Items (which already contains the PR_INTERNET_MESSAGE_ID stamped by Exchange) is synched down to your OST. So when you view the message in the Sent Items in the OST, it contains the same PR_INTERNET_MESSAGE_ID as it did in Online mode.
3. In Cached mode, when you send a message, it goes to the Outbox and is copied to the Sent Items BEFORE the message is submitted to Exchange. Since Outlook does not stamp the PR_INTERNET_MESSAGE_ID, the copy in the Sent Items does not contain this property. So when you view the message in the Sent Items, the PR_INTERNET_MESSAGE_ID is not present. The next time you synch, the message in your Sent Items is synched to your Exchange store, and then gets stamped by Exchange with a PR_INTERNET_MESSAGE_ID. The item in the Sent Items in the OST remains unchanged (no PR_INTERNET_MESSAGE_ID). (However, if there is a subsequent change to the online message in the Sent Items that causes it to synch back to the OST, the message in the OST will then contain the PR_INTERNET_MESSAGE_ID property.)
4. If you then switch to Online mode after sending the message in Cached mode, the message in your Sent Items in the mailbox (not the OST) contains PR_INTERNET_MESSAGE_ID. The message in the Sent Items in the mailbox contains a different PR_INTERNET_MESSAGE_ID than the received item in the Inbox, because it was submitted to the store (during synch) as a separate message.

Steps to reproduce this issue : ( we can see the value of PR_INTERNET_MESSAGE_ID using MFCMapi )

1. Message sent/received in Online mode and viewed in Online mode - PR_INTERNET_MESSAGE_ID is present on message in Sent Items and is the same as PR_INTERNET_MESSAGE_ID of the message in the Inbox
2. Message sent/received in Online mode and viewed in Cached mode - PR_INTERNET_MESSAGE_ID is present on message in Sent Items and is the same as PR_INTERNET_MESSAGE_ID of the message in the Inbox
3. Message sent/received in Cached mode and viewed in Cached mode - PR_INTERNET_MESSAGE_ID is NOT present on message in Sent Items
4. Message sent/received in Cached mode and viewed in Online mode - PR_INTERNET_MESSAGE_ID is present on message in Sent Items and is not the same as PR_INTERNET_MESSAGE_ID of the message in the Inbox

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