No way I'm going to believe this unless you explain me how it works.
We offer 3 different sending modes: - tracking only -> the email can't be modified after sending but you get a notification when the recipient opens the mail - attachments only -> you can not modify the emails content but you can modify / remove attachments - full unsend -> you can modify / remove contents of the emails after its been sent / opened + same for attachments
Although the site also says it can also remove your attachments, so maybe the "email" itself is entirely hosted by unsend.it, which sends a link (probably a token) to the recipient on behalf of your account?