Outlook 2016 OST file behavious

Matthew Beechey - Mobius 1 Reputation point

I have a client with a 26GB mailbox on 365. The strange thing is his PST file keeps growing to the 50GB limit in a very short period of time. Averaging about 10-14 days.

The only oddity might be the way he does his email filing. When he's actioned an email he soft deletes it. Then when he has time he files emails from his deleted items into folders it they require keeping.

Is there something funny in the handling of emails in an OST file when moving in and out of deleted items that they become duplicated in the OST file. I would have thought any moving of an email between folders just changes the reference to the location and doesn't require duplicating the email stored in the OST or am I wrong?

Any help on this would be great as I'm having to delete his OST file so it re-downloads at least once a fortnight at present. I'm fairly certain on completion of the download the OST file is the correct size but I will double check today just incase he is syncing someone elses calendar who maybe has bad habits of filing meetings with large attachments (I've seen someone with a 5GB email booking before on an Exchange server where the limits were turned off completely)

Any other pointers to what might be going wrong?? To be honest I'm not 100% sure I've checked if his version has all updates but in the environment in question I'd expect it to be up to date.

Outlook Management
Outlook Management
Outlook: A family of Microsoft email and calendar products.Management: The act or process of organizing, handling, directing or controlling something.
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  1. ChristyZhang-MSFT 11,826 Reputation points Microsoft Vendor

    Hi @Matthew Beechey - Mobius ,

    Welcome to our forum!

    In order to further confirm the problem, please provide information about the following problems:

    • What's the version of your Outlook? (File > Office Account > About Outlook). Please make sure you have upgraded your Outlook to the latest version.
    • Do you use manual archiving or automatic archiving?
    • You mentioned that duplicate messages will appear after moving deleted items, right? If so, how did you view it, by searching the mailbox and finding two identical emails?

    According to your description, I noticed that you seem to have many concerns about the large mailbox size. As far as I know, exporting mail to backup is also a good way to reduce the mail size. For more information, please refer to: Export or backup email, contacts, and calendar to an Outlook .pst file.

    If the response is helpful, please click "Accept Answer" and upvote it.
    Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.

  2. Gopal Agarwal 401 Reputation points

    Hi @Matthew Beechey - Mobius ,

    I think you have a large amount of Public Folder Favorites, or you are accessing shared mailboxes. If you have configured Cached Exchange Mode, then shared items also keep downloads in the OST file that can increase the size of OST so quickly.
    I suggest you turn off the Cache Mode for shared mailboxes and access it through Outlook Web.

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