If you’ve been using Microsoft Outlook chances are just like me you’ve missed out on a wonderful little tool called “Clean Up Conversation”. It’s a brilliant gem.
This is especially useful if you’ve subscribed to mailing lists or even in corporate scenarios where there are a number of mails back and forth from various people on the same topic.
Conversation Clean Up evaluates the contents of each message in the Conversation. If a message is completely contained within one of the replies, the previous message is deleted.
In case you’re worried about the kinds of mails it will remove there are various options you can control such as deciding where the deleted message go, keeping unread, categorized or flagged messages etc.
I just cleared some 2000 items from my inbox 
best of all you can do this for folders with sub-folders within them. There is a wonderful help file out here http://office.microsoft.com/en-us/outlook-help/use-conversation-clean-up-to-eliminate-redundant-messages-HA101867736.aspx which describes the features in detail.
