Incomplete import of large .PST file into new mailbox.

New Contributor

Following a move from the UK to France, I was required by Microsoft to build a new tenant in France, migrate my data and close the old tenant.  As I only have four users, I decided to export the mailboxes to individual .PST files and import them to the empty mailboxes in the new tenant.


The new tenant is a Microsoft 365 Business Standard.


Everything has worked well but my own mailbox, just short of 24GB has come back with a size of about 19GB.  I’ve tried re-running the import with ‘don’t import duplicates’ option but still have the same result.


I’d be very grateful for any thoughts/suggestions please.


kind regards,



4 Replies

Hey @MeMarkPiecrust 


Exporting and importing PSTs can be a pain! PST import and export via outlook client is not that reliable for large PSTs. In some cases, I have seen size differences and I have not been able to explain what was missed!

Here is something you can try:

connect to exchange online PowerShell for source environment:


Run the following command for your mailbox:

Get-MailboxFolderStatistics -Identity "" | select Name,FolderPath,recoverableitemsfolder,ItemsInFolder,FolderAndSubfolderSize | export-csv SourceUser.csv -nti

Run the same command for the destination tenant. Compare the two sheets and see in which folder you are missing out on emails. Next, you can export-import only those folders via pst.



Many thanks Harveer, unfortunately I can’t try that. I was unable to complete the creation of the new tenant until I closed the old tenant.

Kind regards,


@MeMarkPiecrust Oh, sorry, i guess i should have read it twice before dropping a response. Now that you don't have access to the old tenant, I guess the only validation you can do is whether the PST imported properly or not. Unfortunately, there is no way to verify if the PST import you did was actually successfully imported or it went through a partial import and showed you a successful message. Here are a few random thoughts:

1. Not sure if this would work but give it a shot, Since you have already imported it twice with no difference; Map the PST file you have as another data file in outlook. Next, right click against each folder to look at item counts in each folder. The default account configured in outlook does show the item count; not so sure about the pst though, If it does you should be able to get an idea which folder is missing files.

2. Try importing the same pst into a different account with outlook in 'online mode' and see if you get different results. Maybe use a shared mailbox for testing.

3. Another approach you can try to import psts other than outlook, if you are considering starting over :

4. If you do plan to start all over again, rather than importing the pst file you can choose to drag and drop one folder at a time from the mapped pst.


and finally may be if its not that critical, don't bother fixing it, just keep the pst around and use it when you feel there are files missing by simply mapping it in outlook rather than importing it to browse through old emails.




Some great ideas there. Many thanks. I’ll let you know how I get on.

Kind regards,