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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?

Это отзыв для предыдущей версии этого дополнения (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.