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Environment: MS Office LTSC Pro Plus 2021 under Windows 11 Pro 64.

This is a follow-up to my question on StackOverflow: In MS Outlook VBA, how to find hidden data in a MailItem.

I've been studying the contents of my Outlook pst file. It contains e-mails sent and received since 2004, 19 years.

I use outlook only for e-mail. I don't use its calendar or other features, although sometimes it stores events in the calendar when someone asks for a meeting and I respond. I assume that the contents of the pst file not directly attributable to e-mails contained in it is negligible.

On June 1, I output the properties of all items in e-mail folders of the pst file to MS Access. This was 300,723 items. The sum of the property MailItem.Size for all items is 9.5 * 10^9. Unfortunately, I did not save a copy of the pst file at that time, but the size of my backup file on May 27 is 15.7 * 10^9 and the observed size of the file on June 9 was 15.8 * 10^9. So I estimate that the size of the pst file on June 1 was 15.7 * 10^9. This means that the size of the file is 6.2 * 10^9 or 65% larger than the sum of the MailItem.Size of all the items.

In the StackOverflow question linked above, in the section Hidden data, I noted three strange observations. The first was that Outlook appears to be saving hidden copies of e-mail attachments that I removed. The third was that Outlook appears to be saving hidden copies of e-mails that I permanently deleted (i.e., not just moved to the folder "Deleted Items").

The new observation that the size of the pst file is 65% larger than the total MailItem.Size of all the e-mails found in its folders appears to be consistent with the previous observations that removed attachments and deleted e-mails seem to be hidden somewhere in the file. But before assuming that that is correct, I need to know what the storage overhead is for e-mails in Outlook. In other words:

  • If the total MailItem.Size of all e-mails stored in a pst file is x, with no attachments removed and no e-mails deleted from the folder "Deleted Items", and the size of the file is y, what the expected size of the ratio y/x?

Assuming that the answer to that question is far less than 1.65, I am left with the questions asked in the StackOverflow question: How can I find where in the data structure of the file those removed attachments and deleted e-mails are hidden, and how can I remove them from the file?

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  • Please use this site in the way it is designed to be used. When you have a second question, post a second question. When you have an update to your initial question, or to your initial answer, use the EDIT buttons to add the info.
    – music2myear
    Commented Jun 17, 2023 at 2:12

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PST file has its own storage system, not that different from FAT32 or NTFS. When messages or attachments are deleted, you can expect to have empty sectors - Outlook won't rewrite multi-GB PST file is you delete a 10kB email. The data is not immediately returned to the host OS (Windows) - the sectors used by the deleted email will be marked as empty and will be reused later. It is expected that the storage becomes fragmented.

If you want to reclaim the storage space, you can compact the PST - right click on the top PST file folder, select "Data File Properties", "Advanced", "Compact Now".

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  • Thank you Dmitry. I've followed your instructions and reported the results in another answer here, where I ask another follow-up question. Could you take a look and see if you have an answer to that?
    – NewSites
    Commented Jun 13, 2023 at 0:36

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