Splitting an Online PST file can be done using the built-in tool provided by Microsoft called "New-MailboxExportRequest" in Exchange Management Shell. Here are the steps to split an Online PST file:

Open Exchange Management Shell by running it as administrator.

Connect to Exchange Online PowerShell by running the following command:

Connect-ExchangeOnline -UserPrincipalName <UPN>

Replace <UPN> with the user principal name of the mailbox containing the PST file you want to split.

Create a new folder where the split PST files will be saved. You can do this by running the following command:

New-Item -Path "<FolderPath>" -ItemType Directory

Replace <FolderPath> with the path of the folder where you want to save the split PST files.

Run the following command to split the PST file:

New-MailboxExportRequest -Mailbox <MailboxName> -FilePath "<FolderPath>\<PSTFileName>.pst" -ContentFilter {(Received -lt '<Date>')}

Replace <MailboxName> with the name of the mailbox containing the PST file you want to split. Replace <FolderPath> with the path of the folder created in step 3. Replace <PSTFileName> with the name you want to give to the split PST file. Replace <Date> with the date before which you want to split the PST file.

For example, if you want to split the PST file for all emails received before January 1st, 2022, you would run the following command:

New-MailboxExportRequest -Mailbox john.doe@contoso.com -FilePath "C:\PSTFiles\john.doe.pst" -ContentFilter {(Received -lt '01/01/2022')}

The split process will start, and you can monitor its progress by running the following command:

Get-MailboxExportRequest

This will display the status of the split process. Once it's complete, the split PST file will be saved in the folder specified in step 3.

By following these steps, you can split an Online PST file using Exchange Management Shell. Note that you need to have the necessary permissions to run these commands, and this method is only applicable for Exchange Online.

Greetings,
Peter