Manually export/import PST files in classic Outlook via File > Open & Export > Import/Export. Choose "Export to a file" to backup data, or "Import from another program or file" to restore it, selecting .pst as the format. Ensure "Include subfolders" is checked during import. 
How to Export PST (Backup)
  1. Open Outlook and click File in the top left corner.
  2. Select Open & Export > Import/Export.
  3. Choose Export to a file and click Next.
  4. Select Outlook Data File (.pst) and click Next.
  5. Select the folders (Inbox, Calendar, etc.) you want to back up.
  6. Click Browse to choose where to save the file and name it, then click Finish. 
How to Import PST (Restore)
  1. Open Outlook and go to File > Open & Export > Import/Export.
  2. Select Import from another program or file and click Next.
  3. Select Outlook Data File (.pst) and click Next.
  4. Click Browse to select the file to import.
  5. Under Options, choose how to handle duplicates (usually "Do not import duplicates" is best) and click Next.
  6. Select the folder to import into, ensure "Include subfolders" is checked, and click Finish. 


Supporting Links: How to Import Export PST by Microsoft