User found an old pst file which is around 15GB and imported to his exchange mailbox. It imported fine. Outlook client starts syncing the emails to exchange. It stalls and then the user will need to reopen the client for it to sync again. The client doesn't crash and still continues to receive new emails but it just doesn't sync the old emails. Sits on Updating folders but doesn't do anything. Shows no sync or conflicts in the client. No errors on progress and Outlook network connection seems fine.
Check for: Size limit on the user mailbox, either set at the database or mailbox level. Bandwidth limit between user and server.
Consider: Disabling cached mode, to have just a "1 way" data stream at the moment, eg) user workstation > Exchange, rather than user workstation > Exchange > user .ost file
Importing the pst using the New-MailboxImportRequest powershell command, rather than through Outlook.