You are composing an e-mail message. You attach a file to the e-mail message. After composing the e-mail, you press the Send button. However, Outlook 2010 prompts the following error message.

If you click the Yes button, Outlook will send the message. However, re cipients of the e-mail will not be able to open the attachment because it is blocked. If you click the No button, you can remove the unsafe attachment.
Which of the following is a cause of the issue?