1) Click on the File tab.
2) Click Open & Export and choose Import/Export.
3) Choose Export to a file in the action to perform options.
4) Click Outlook Data File (.pst), and click Next button.
5) Select the name of the email account to export, and make sure to tick the checkbox Include subfolders. Click the Next button.
6) Click Browse, and then select the location path where you want the .pst file to be saved.
7) Type the name that you want to use for the new .pst file in the File name box, and then click OK.
8) Under Options, specify what to do when exporting items that already exist in the file if you are exporting it to an existing Outlook Data File (.pst).
9) Click Finish.
10) Outlook will begin the exporting process.