When you evaluate SendMail with “Password” -> Automatic for a particular user account, you will be asked to provide your credentials in the login dialog. You can choose to save these credentials.
Your email credentials will be encrypted using keys stored on your computer’s hard drive. The encrypted credentials will be transmitted to the Wolfram Cloud, and may only be accessed after you have authenticated using your Wolfram ID.
If you have chosen to save your email credentials, you can update your stored password by providing the new password as an option to SendMail:
You can delete your stored credentials by evaluating SendMail with “Password” -> None: