Emails can be unsent within a very short period after hitting the send button. An undelivered email can be canceled immediately after hitting send. Gmail can unsend emails for up to the amount of time set in its undo send options.
The undo send function in Gmail must be enabled when the email was sent in order to unsend it. This function can be enabled from the labs tab under the Gmail settings gear icon. Ensure "enable" is selected for the "Undo Send" option. Now emails can be unsent for a few seconds after the send button is pressed.
AOL and other email carriers allow the cancellation of undelivered messages only.