Using Thread Compressor

Clean up your mailbox

If you use email to conduct extended conversations but aren't diligent about deleting messages that you've read and replied to, you'll appreciate a utility called Thread Compressor. Thread Compressor cleans up and consolidates sets of email messages that relate to a common topic. At work, I subscribe to several distribution lists (DLs) that address topics ranging from Windows to Exchange Server, and I rely heavily on Thread Compressor to keep my Inbox in order.

Installing and Configuring Thread Compressor
You can download the Thread Compressor installation file,, from Grab it while you can*the Web site at which I got the tool ( is now shut down. Microsoft employees have access to a version of Thread Compressor that comes in a Windows Installer package that automates the installation; because this version is difficult to come by outside of Microsoft, I also explain how to install Thread Compressor manually. Note that Thread Compressor runs only with Outlook 2002 or Outlook 2000; it doesn't work with Outlook Web Access (OWA).

Before you install Thread Compressor, make sure that Collaboration Data Objects (CDO) for Microsoft Outlook is installed with Microsoft Office XP or Office 2000 on your system. Office doesn't install CDO by default, so you must run the Office Setup Wizard from the Office CD-ROM or from your network installation source. Select Run from My Computer under Features to install, Microsoft Outlook for Windows, Collaboration Data Objects, as Figure 1 shows.

Thread Compressor is an Outlook COM add-in. You must copy the Thread Compressor file, threadc4.dll, to the \%APPDATA%\Microsoft\AddIns\ folder (where %APPDATA% is a system environment variable such as C:\Documents and Settings\Username\Application Data). To distribute Thread Compressor within your company, use a simple batch file, such as

Copy threadc4.dll %APPDATA%Microsoft\AddIns\

When the Windows Installer package configures Outlook for the COM add-in, it copies the threadc4.dll file to \ProgramFiles\Thread Compressor. I've experienced problems when running the file from that folder, so I copy it to \%APPDATA%\Microsoft\AddIns. After you use Windows Installer to install Thread Compressor, you must restart Outlook.

The next step is to enable the Thread Compressor add-in as part of Outlook. If you're installing the Windows Installer version, the installer enables the add-in for you. To enable the add-in manually, launch Outlook and select Tools, Options. In the Options dialog box, click the Other tab and click COM Add-Ins, as Figure 2 shows. In the next dialog box, click Add and browse to the \%APPDATA%\Microsoft\AddIns directory. Select the threadc4.dll file, click OK, then click OK again to initialize Thread Compressor. Click OK twice to close the Options dialog boxes.

Using Thread Compressor
Now you're ready to run Thread Compressor against DL messages or sets of messages about a common topic. Select a folder or the root of a folder tree (Thread Compressor works on subfolders). From the Tools menu, click Compress Threads, as Figure 3 shows. In the dialog box that appears, click the Options tab to configure the tool. As Figure 4 shows, I selected the Use Deleted Items Folder check box so that Thread Compressor wouldn't permanently delete messages. (Note: If you choose not to use the Deleted Items folder, you might not be able to recover messages that Thread Compressor deletes.) Compress Sub-folders too is enabled by default. I set the System Resource Usage to High and Logging to Low.

To configure Thread Compressor so that it won't delete messages from certain users (e.g., users whose responses you value but who often reply without including previous message content), click Exceptions and select names from the Global Address List (GAL). You can also configure the tool so that it won't delete certain attachment types (e.g., code). By default, Thread Compressor won't delete most graphics files. To configure Thread Compressor to compress a different folder, click the ellipsis (...) button and select a folder.

Click Start, and Thread Compressor will begin to do its work. If you've installed Outlook 2002 (as part of Office XP) or Service Release 2 (SR2) for Microsoft Office 2000, you'll receive a warning message, which Figure 5 shows. SR2 prevents viruses from accessing your address book programmatically. Select the Allow access for check box, and the tool will run.

I've encountered a problem with Thread Compressor that occurs when the tool needs to run longer than I've configured. If Thread Compressor runs out of time, Outlook might stop responding, and you might need to reboot your machine. As Figure 5 shows, I gave the tool 2 minutes, but you might want to bump this up to the maximum of 10 minutes when you run Thread Compressor for the first time, especially if you have many subfolders that you want to clean up.

When you run Thread Compressor, the tool presents the message Are you REALLY sure you want to delete these XX messages? You'll save <##>K of space. This step provides an extra precaution, especially if you didn't configure Thread Compressor to use the Deleted Items folder. Click Yes, then click OK.

To see a summary of the messages that the tool processed, click the Status tab. You can also click the About tab and select the option to hide the Thread Compressor splash screen when the tool starts up. Back on the Options tab, you'll find another important option: the Use QuickCompress check box. When you select this check box, Thread Compressor runs with a minimal interface that requires little intervention. After Thread Compressor cleans up messages, it performs a countdown, as Figure 6 shows, before it closes. The countdown gives you a chance to restore the full interface, look at the logging information, and change other options.

You can also run Thread Compressor against messages in personal folders (PST) files, public folders, and subfolders. Unfortunately, from what I've determined, Thread Compressor doesn't remove threaded DL messages that originate from the Internet. Nevertheless, the tool does a great job of keeping my mailbox tidy. When I receive the familiar "your mailbox is over the size limit" warnings, the first thing I do is run Thread Compressor and empty my Deleted Items folder.

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