Question about PST and Exchange Online mailbox sizes

Hi,@X-Box-11-2021

Thanks for posting your question in the Microsoft Q&A forum.

According to your description, when importing a PST file into a shared mailbox, the mailbox size appears smaller than the original mailbox.

There may be several reasons for this phenomenon:

1.The PST file exported by the third-party cloud backup you are using may contain metadata such as views, rules, categories, and indexes. It is recommended that you use Outlook to export the PST file and then check the size of the PST file.

2.If a PST file contains a mailbox item that is larger than 150 MB, the item will be skipped and not imported during the import process.You can refer to this link:https://learn.microsoft.com/en-us/purview/importing-pst-files-to-office-365#is-there-a-message-size-limit-when-importing-pst-files-using-drive-shipping

We recommend that you check your mailbox for lost emails, if there are no lost emails or import error message alerts, then there is no need to worry.

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