how to cancel an e.mail already sent?

Impossible. It is already on the server the other person uses and it may already be on the other user's computer if they have connected to the server.

People have been mistakenly sending emails since email was invented. Welcome to the electronic age.

The best thing you can do, if you don't like what was in the e-mail, is send a followup e-mail with a subject line similar to the old one, except saying

[old subject line] please disregard prior e-mail.