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I issue this command to export the mailbox of a user:

New-MailboxExportRequest -Mailbox "user" -FilePath "\\server\share\user.pst"

After 2 minutes or so, this file have been generated: enter image description here

Notice the size of the export file, and now this is the actual size of the mailbox on the server as seen from outlook:

enter image description here

Where does this extra ~100MB in the export file comes from?

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It is likely the additional items that are exported, such as the recoverable deleted items (dumpster).

I would mount the .PST file in your outlook profile and check each folder to see exactly where the additional mail items are.

A mailbox export will export EVERYTHING within the mailbox, including items that aren't viewable to the user.

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It may also be related to message compression. I'm assuming that you'd lose that on a mailbox export. - – joeqwerty Dec 17 '12 at 16:59
@joeqwerty ah, very interesting, I had not seen that before. I would imagine you'd lose the compression on an export too. – HostBits Dec 17 '12 at 17:08
Agreed. OP can check this with: Get-MailboxStatistics <Identity> | Format-List StorageLimitStatus,TotalItemSize,TotalDeletedItemSize,ItemCount,DeletedItemCount – TheCleaner Dec 17 '12 at 17:19

I have found the reason.

The export contains all the deleted and purged messages of the mailbox, even after the "Deleted Elements" have been emptied in outlook. So once I opened the archive into outlook, I saw all my emails that I have deleted and purged months ago.

That's a good thing I learnt, thanks to you guys!

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If the answer I provided you was what you were looking for, please mark it as accepted. – HostBits Dec 17 '12 at 19:26
Actually, you should probably go back through your questions and mark a lot of answer as accepted. You have a really low accept rate and people will be hesitant to answer your questions. Plenty of your questions have some really helpful answers (including at least one of mine), so there is no reason not to go back and accept those answers. – HostBits Dec 17 '12 at 19:43

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