No its not possible. Unsend is a feature only with proprietary e-mail systesm and only within the system. The only e-mail system I know that has it is AOL and then you can unsend only if the recipient hasn't opened it.
there is a feature in outlook to "unsend" an email but all it does is sends a new email to the person and ask them to ignore the previous email, something you could do yourself, so its a very useless feature.
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