Outlook 2003 introduced a new file format which supports larger data file size (default is 20 GB) and unlimited messages per data file. This new format is commonly known as "Unicode". To determine which format your pst file is, you need to look on the data file properties dialog.
The pst file needs to be in your profile to check the format.
If Outlook is using an ANSI pst file and you want to change to a Unicode format, see Convert an ANSI PST to a Unicode PST. Note that you cannot control the pst format for IMAP accounts; Outlook chooses the format.
Outlook 2010 and up
In Outlook 2010 or newer, go to File, Account Settings, Data File tab. Double click on the pst file you want to check to open the Outlook Data File property page.
If the format is Outlook Data File, you are using a Unicode pst file.
When the Format field includes "97-2002" in the format name, it is ANSI format with a file size limit of 2GB: Outlook Data File (97-2002).
Outlook 2007 and Outlook 2003
In Outlook 2007, you can check the file format either from Tools, Account Settings, Data File tab or from the File, Data File Management menu. Double click on the pst file you want to check.
In Outlook 2003, check the file format using the File, Data File Management menu. Double click on the pst file you want to check.
In both Outlook 2007 and Outlook 2003, the dialog will look identical to Outlook 2010's dialog but the format is called Personal Folders File for a Unicode pst file. When the Format field lists Personal Folders File (97-2002), it is ANSI format with a file size limit of 2GB.