This article applies to the ANSI pst format used by Outlook 2002 and older. While it is possible to create an ANSI pst in Outlook 2003 to 2013, the default pst format in the newer versions of Outlook is Unicode, which supports up to 1TB pst files (50GB is the recommended max.)
Microsoft Outlook limits the size of the older format Personal Folders (*.pst) files and offline folders (*.ost) files to 2GB. If a new or changed item increases the size over than limit, you will not be able to use the .pst or .ost file. Even if you delete items from the server mailbox, the .ost file will not synchronize.
If you need to use an older .pst file in Outlook 2013, 2016, or newer and plan to leave it in the profile, it is recommended you import the pst to a new pst file. If you aren't sure if the .pst is the older ANSI-format, check the .pst properties. In Outlook 2003 and newer, an ANSI pst is identified as Outlook Data File (97-2002).
Beginning with Outlook 2003, larger PST files are supported. However, if you upgrade from an older version you need to create a new PST, the older PST format is not automatically converted or upgraded. See Convert an old PST to a Unicode PST. If the old PST won't open in Outlook you will need to repair it before you can convert it.
How to check the data file (pst) format
- Right click on the top level folder and choose Properties (Data File Properties in Outlook 2010 and newer)
- Click Advanced to open the data file dialog.
- If the data file is listed as Outlook Data File and you are using Outlook 2003 and up, its a Unicode data file.
- If the data file is labeled Outlook Data File (97-2002) its an older ANSI format and is limited to no more than 1.9 GB in size and no more than 65,000 items per folder (often less).
Shrinking a file that's over the limit
Outlook has no built-in way to reduce a PST or OST file that has gone over the limit. However, Microsoft provides a tool, PST2GB, which you can download to restore the file to operability. Note, however, that the utility truncates the data from between 25 and 50mb and the truncated data is not recoverable. The remaining data under 2GB should be restored.
If you want to attempt repairs yourself, use a tool called a hex editor to edit the data in the file directly. Every Windows shareware site has a selection of hex editors. Eugene McCarthy of FAO Headquarters in Rome recommends the free iHex editor, which loads only 64kb of the file at a time, making it ideal for tinkering with extremely large PST files.
Make a backup of your PST or OST, then open the original file with the hex editor (available for download from any Windows shareware site). You'll want to remove a few characters from the middle of the file, then save it and try to run the Inbox Repair Tool (Scanpst.exe) to repair the damage you did with the hex editor. If Scanpst can repair the file, you should be able to open it Outlook again. If not, try removing some additional characters from the file.
If this process works, with any luck, you'll lose only one or two items. You may be able to recover 100% of the data by repeating the process with a new copy of the file, but removing the data from a different area.
Preventing PST files from growing too large
Beginning with Microsoft Office XP Service Pack 1, Outlook provides better protection against Personal Folders .pst and offline folders .ost file growing past the 2GB size limit. Once an PST or OST file reaches 1.82GB, users will not be able to add new items. This MSKB article contains more information: Mail Delivery to Inbox Stops When the Personal Folder or Offline Folder Is Full