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What if others are using Outlook or web mail like Yahoo and Gmail? Such function is stopped at once then, isn't it?

This review is for a previous version of the add-on (1.01.0008). 

This partly correct. Basically Boomerang just makes it more comfortable to add changes to an e-mail and sending it to a next person. If the additional Headerinformation is treated right it will work even if an Outlook-User is in between.