What are the underlying mechanisms allowing us to unsend an email?
The e-mail system has no way of manipulating already sent e-mails, no.

The only services I've seen which allow you to "unsend" an e-mail simply delay the e-mail being sent for a user-determined amount of time and then allow you to cancel the send before it happens. 

Alternatively, I've seen some services allow you to "unsend" e-mails if the recipient is using the same e-mail client as the sender, as both sides are controlled by the same service. Although, once again, this is handled by the service provider, not by the e-mail system itself.