SmtpClient.SendAsync - Stop application after the call

I am building winforms application that has to send emails. I want to be able to call SmtpClient.SendAsync method and after that allow to user to close the app immediately (I am not interested to get sending results in callback function). If user stops application, the emails are not sended....If application stay running, the emails are sended as expected... Any suggestions, how can i pass all relevant details to smtp server and close my app?

Thanks