Outlook Thread Compressor
deletes unnecessary email
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Thread Compressor is an add-in to Microsoft Outlook, which removed unnecessary emails from a "thread" - reducing the amount of storage required (maybe keeping your mailbox within its size quota) and reducing the number of emails you need to read.
 
TC was developed inside Microsoft from 1999 onwards, and attracted a large following (up to 30,000 users) but has never (officially) been made available externally, due to the fact that it will delete data unless it is configured not to. I've decided to share it more widely now.
 
Let me say that again: Thread Compressor, as it is configured by default, WILL DELETE DATA FROM YOUR INBOX.
If you choose to download it and use it from here, you do it with the author's blessing, but it's completely at your own risk and Microsoft cannot be held responsible for what it does.
 
If you're in any doubt about this, then do not use this tool.
 
There are several articles online about using TC:
 
 
How does it work?

The premise is that most responses to emails in Outlook will include the original. So if Bob sends a question to Jenny, who replies to Bob and copies Carl, who then replies to both with the answer... then Carl's reply contains everything that the previous 2 messages had, so we don't need to keep them as well.
 
You'll find that TC is only really effective on internal mail - since it uses Outlook's conversation threading properties that are preserved for Outlook/Exchange messages but don't carry over onto mail that goes outside. If you use TC and you start seeing messages that you think *should* be compressed, but haven't ... then this is the likely reason.
 
Another reason for not compressing is that TC will keep messages with attachments - so if Bob's mail to Jenny had a document attached, and Jenny replies, Outlook will remove the document. TC will therefore preserve Bob's original mail within the thread.
 
Once you're confident in TC, you may choose to have it actively delete content. Otherwise, you could have it move "compressed" items to your Deleted Items folder or some other folder you may create (in a PST file, for example). There's also an option on the Advanced tab to have it not remove mail, but just mark it as read. Or to keep various flavours of mail (like high priority ones). Have a play. You'll figure it out.
 
Brief explanation of usage
 

 
Here's the standard window. TC starts in whichever folder you're viewing when you launch it, though you can use the "..." to the right of the Selected Folder, to choose a different one.
 
If you configure TC to use Deleted Items folder (or some other folder that you can select), it will move "compressed" items in there rather than outright deleting them.
 
QuickCompress is a feature which means it starts compression automatically. You may want to try that out if you use it regularly, but don't worry about it at first.
 
The Attachment Types section allows you to configure TC to not keep messages with attachments - eg if Bob mails Jenny and has a VCF contact attached in his email signature, TC would elect to keep Bob's mail in the thread because it has an attachment. If you wanted TC to not keep mails with just those types of attachment, then add VCF to the list.
 
Exceptions. Sometimes there are people whose mail you just want to keep regardless of whether it's in a thread or not. Add their names to the list.
Compatibility
 
This version (4.2.030) is the last that will ever be developed. It works on all versions of Outlook and all versions of Windows post-2000. The steps to install it are different depending on which version of Windows you're running (ie if you're on Windows Vista or Windows 7, you need to run the install process from an elevated command prompt).

INSTALLING

* Firstly, download the ZIP and save it locally.

* Create a folder you'll find again - I'd suggest C:\Program Files\Thread Compressor or similar.

* Start a command prompt - WindowsKey-R then
cmd <enter> (though if you're on Vista or Win7, just press WIndowsKey, type cmd, then right-click on the cmd icon and choose "Run as Administrator")

 
In the command prompt, type:
 
cd c:\program files\thread compressor (or wherever you put the files)
regsvr32 comdlg32.ocx
regsvr32 msflxgrd.ocx
regsvr32 tabctl32.ocx
regsvr32 threadc4.dll
 
Download the latest CDO file from here, save it somewhere, expand it out and run the install from the ExchangeCDO.msi file.
 
Now start Outlook: how you actually install the addin will vary depending on your version of Outlook, but try:
 
Tools | Options | Advanced | Add-ins,
or Tools | Trust Center | Add-ins | [then hit Go to manage COM add-ins]
 
and add the threadc4dll file manually. If it's successful, you should see Comrpess Threads on the Tools menu, and you'll get a splash screen next time you start Outlook. 
 
USING
Should be pretty self-explanatory: launch from Tools | Compress Threads menu. You may find that the first time you do this, Outlook will hang - if it does, kill OUTLOOK.EXE from Task Manager and try again - usually that resolves the "initial run" issue. 
 
If you want to find out what any of the options do, make a copy of your inbox folder, navigate to it, then launch TC from there and you're free to play to see which messages it would delete.
 
FEEDBACK/QUESTIONS
I can't guarantee that I'll be able to respond to questions for support, but if you have feedback or need help with anything then please feel free to send me email.
 
-- Ewan