Microsoft Outlook stores all the data in a single file known as Personal Storage File (PST). In other words, it is a data file used for the storage of Outlook e-mail objects, such as contacts, attachments, appointments, contacts, journal, tasks, events, etc. Each version of Outlook has different format. These formats are of two types- ANSI and UNICODE. Versions prior than Outlook 2002 uses American National Standards Institute (ANSI) format whereas later versions of Outlook employs UNICODE format.
Along with different format, different size limit has also been set by Microsoft. Once the set size limit gets exceed, addition of no more data is allowed in a PST file.
Size limits for different versions of Outlook:
Outlook 2002 and prior versions of Outlook allow you to store data only up to 2 GB of data. Outlook 2003 and 2007 have size limit of 20 GB, whereas Outlook 2010 has file size limit up to 50 GB that can be increased or decreased by applying changes to the Windows registry settings.
Once the Outlook PST file approaches the set size limit, it fails to add more data in it. Moreover, Outlook starts behaving abruptly and can even get corrupt. So, it is always advisable not to store unwanted data of Outlook e-mail objects in a PST file.