An Outlook user discovered the Clean Up Folder command and had a question about it:
I decided to right-mouse button click on one of the folders in my mailbox to mark the messages all read when I noticed another option in the popup that said, “Clean up Folder”. I decided to click on it. It said that it would remove “redundant emails”. What does Outlook consider to be redundant?
Clean Up removes redundant messages in a thread of folder. What is a redundant message? Redundant messages are the previous messages in a thread that are also quoted in the replies. In a long email thread, you could end up several copies of the early messages and the Clean Up Folder command moves these redundant messages in the current folder (or current folder and subfolders) to the designated "Cleanup" folder, leaving you with all the content but fewer messages. It will not remove true duplicated messages; it will not trim quoted text from replies.
The Clean Up folder (and subfolder) commands will clean up all threads in a folder (and subfolders, if that option was selected), while clean up conversation will remove the older messages from the selected thread.
Clean Up will not remove duplicate messages. It's usually faster to use a "duplicate remover utility", however, if the duplicates are due to importing or re-downloading messages, "you can add the Modified date field to the view and sort by it". Everything imported or downloaded later will have the same modified date. Select them and delete.
If you didn't select a clean up folder, Outlook will use the Deleted Items folder.
By default, Outlook won't clean up categorized, flagged, or digitally signed messages. Change these settings in File, Options, Mail.
In addition to Clean Up Folder, Outlook can also Clean Up Conversations, removing older messages in a thread that are quoted in newer messages. Right click on any thread and choose Clean Up Conversation in the context menu or look for it on the Clean Up button in the Delete Group on the Home tab. Clean up Folder and Clean Up Folder & Subfolders commands are also on the Folder tab.
When you use Clean Up, a dialog box will remind you that redundant messages will be moved to the designated folder (unless you ticked the Don't show this message again box). Click the Settings button on the dialog to open the Options dialog to the Mail page. Clean up options are partway down the page, where you can choose a different folder and other options.