If you have an extremely large pst file, exceeding 4 GB, you need to consider archiving your email. Archiving stores the emails that you may not need daily, but do not want to delete. The program will setup a separate pst file, called archive, in the same folder as the original pst file is kept and move all the data you select. This will reduce the size of the original pst file to a more reasonable and stable file.
If you need to retrieve the archived email, all you need to do is import them as you would any other pst file.
To archive email, first determine a cutoff date. This will be the date you tell the program to archive email PRIOR to. For example, if your date is 1/1/2016, the program will archive everything prior to that date.
Go to File, then Archive, (or Cleanup Tools, depending on the version of Outlook), and begin the process. Depending on the size of the archive, the process can take several hours.