For Microsoft Office Outlook clients, there’s a registry entry to limit the physical file size of a .pst or .ost file. This is described in Microsoft knowledge base article 832925, “How to configure the size limit for both (.pst) and (.ost) files in Outlook 2007 and in Outlook 2003.” The article identifies the DWORD entry called MaxLargeFileSize for Unicode .pst or .ost files. For legacy ANSI .pst/.ost files, which have a limit of 2 gigabytes, the DWORD value name is MaxFileSize. The DWORD value in megabytes is found in:

HKEY_CURRENT_USER\Software\Policies\Microsoft\Office\\Outlook\PST\

For the Office_Version, Office 2007 is version 12.0 and Office 2003 is 11.0. Even though the key is called PST, it applies to OST files as well. Follow the knowledge base article if you want to create these limits and even establish warnings thresholds for your users as their .pst or .ost files grow.

You can also assign a maximum mailbox size on the Exchange Server. When a user is configured with Exchange Cached Mode, a slave copy of the mailbox is created and stored on the client machine with the extension .ost. An .ost file has the same limitations as a .pst file. It also maintains a different database structure than the message stores on Exchange. The amount of disk space needed for offline folders (.ost) to host the same content as found in the Exchange mailbox can be anywhere from 10 percent to 50 percent greater. So, what happens if the registry entry described earlier is configured to be smaller than the Exchange Server mailbox quota allows?

I tested this scenario with Microsoft Outlook 2007 in cached mode against an Exchange Server 2007 server. I sent large emails from another user to the test mailbox. The sender was not notified of any issue at all, and this makes sense. I’ll come back to that in a moment. The test recipient mailbox provided two separate pop-up alert messages telling the user of a quota issue. If the mailbox was already open and a message was received that caused the .ost file to surpass the maximum file size, the user received the pop-up alert message shown in Figure 1. If the user logged on and the maximum file size was attained during synchronization, then the Mailbox Cleanup dialog box was displayed, as shown in Figure 2.

Note that the Mailbox Cleanup dialog box did appear in some tests when the user was already logged in when the maximum file size was reached, but this was inconsistent. Both the pop-up alert message and the Mailbox Cleanup dialog box advise the user to reduce the mailbox size. The Mailbox Cleanup dialog box does provide more comprehensive options.

So if the mailbox reached its quota, why didn’t the sender receive notice? Well, the .ost limitation is a client-side configuration. The email sent to the test user was actually received in the Exchange mailbox. The smaller quota assigned to the .ost file didn’t allow Exchange to synchronize with the cached mode Outlook client. Message tracking on the Exchange Server showed that the message was successfully delivered to the test user’s mailbox. The test user was also able to log onto Outlook Web Access (OWA) and see the content that arrived after the .ost maximum file size was reached. After the test user deletes some mailbox content using Shift-Delete and reduces the .ost file size sufficiently below the maximum size allowed by the registry entry shown earlier, then Outlook will download content during synchronization. In some of my tests, it took a few Send/Receive requests from Outlook to show the new content to the Outlook client.