Well, it would depend on what it was, exactly. If it was something you deliberately sent to a whole group, but then thought better of, the best thing to do is apologize to everyone, perhaps explaining the feelings that led to the mail. You can do this by email.
The other common case is that intended to say something nasty about person X to person Y, but instead sent it to a group. Now you need to apologize to Y, and you somehow need to make it up to them: a gift, a social invitation, something. And you need to apologize in person, not by email.