Maximum .OST file size in Unicode is 20GB for Outlook 2007 and 50GB for Outlook 2010/2013/2016. By default the maximum size is set to 20GB. When the .OST file reaches or exceeds this limit, you will encounter one or more of the following errors:

  • Cannot open or load the OST file at all.
  • Cannot add any new data to the OST file.
  • Cannot synchronize the OST file with the Exchange server.
  • Various error messages during the synchronization process.

Read the Knowledge Base article on What Are Offline Folder Files? for more information about the OST file.  

Note: OST file does also include Shared Folders/Mailboxes data and Public Folders data, if profile is configured to Download Shared Folders. You may check that in Outlook under File/Tools > Account Settings > Account Settings > double-click on your Exchange address > More Settings > Advanced.  

You may modify the following UNICODE Registry values to increase the limits:

  • The MaxLargeFileSize registry entry. The MaxFileSize registry entry determines the absolute maximum size that both the .pst and the .ost files can grow to. After this maximum size is reached, Outlook does not permit the size of the file to grow beyond this size.
  • The WarnLargeFileSize registry entry. The WarnFileSize registry entry determines the maximum data that both the .pst and the .ost files can have. After this maximum data is reached, neither the .pst nor the .ost files are permitted to add any more data. However, the size of the physical file may still increase because of internal processes. 

Note: The settings apply to both .ost and .pst files. If you modify these registry values, this can affect .ost files that are used with Cached Exchange Mode, with AutoArchive, and with .pst. files.

To proceed follow the steps below:

  • Open Registry: Start > in Search field type in "regedit" without quotes.
  • Navigate to HKEY_CURRENT_USER\Software\Policies\Microsoft\Office\12.0\Outlook\PST for Outlook 2007, HKEY_CURRENT_USER\Software\Policies\Microsoft\Office\14.0\Outlook\PST for Outlook 2010, or HKEY_CURRENT_USER\Software\Policies\Microsoft\Office\15.0\Outlook\PST for Outlook 2013.
  • Right-click on MaxLargeFileSize in the right pane > Modify > type in value in MB in Value Data box > Ok.
  • Right-click on WarnLargeFileSize in the right pane > Modify > type in value in MB in Value Data box > Ok.

Note: We do not recommend to have OST files bigger than 20GB. 

Alternatively, you may reduce your mailbox size using Outlook Archiving. Read Knowledge Base article on How Does Outlook Archiving Work? for more details.