You definitely can't undo sending an email notification so additional emails will have to be sent when an action is undone, which in the end will create more problems, annoy or confuse users.
MS Outlook has taken this approach, IIRC - you can send a "message cancellation", which is actually just an e-mail with a special header. This, however, requires all the people in the conversation to cooperate, i.e. use Outlook.
Hilarity ensues when someone tries to pretend that a message never existed by cancelling an e-mail sent outside Outlook's garden. (The non-Outlook user will get both the original and the "previous message doesn't exist, okay? Wink wink, nudge nudge" message.)
