Depending on your email provider, you may be able to go to the webmail page and delete them from the sent/sending/whatever folder. But in about a week they'll time out and disappear anyway, so it's not a real problem. (And you''ll probably get an email from the email daemon telling you why the message couldn't be delivered. The usual reasons are no such recipient [you mistyped the To address or the account no longer exists] or the recipient's mailbox is full.)