If you use Outlook and your setup on the POP protocol it's worth backing up your emails regularly.
Here's how to do it
Have a memory stick or external hard drive handy and plugged into your computer.
Open Outlook and locate the PST file first
Click on tools, then Account Settings/ Email Accounts (On older versions of Outlook)
Select File, then Account Settings (More recent versions of Outlook)
Select Data Files
Make a note of where the data file is located, in the example image above (and your's will be located somewhere else) it's located in the C drive on the computer under the Users Folder\LeoN\Documents\Outlook
Close the Outlook when you have established the PST's location
Follow the path to the PST from your file explorer
Once you reach the correct folder simply right-click the PST file > select COPY> then navigate to the external drive and PASTE the file on the external storage unit
It could take some time to copy depending on the PST file size, do not interrupt the process
Once completed; store the external somewhere safe and use it should something happen to your PC or Outlook program
Remember: Regular backups are good practice and could be what saves you when disaster strikes