Essentially what this means is that if you do not select the proper authentication for your incoming POP messages, you will not be able to receive your mail. To change the authentication options in Mail, follow these directions:
- Open Mail and navigate to the Mail menu bar > Preferences (or Command + ,).
- Choose Accounts and select the POP account that is not receiving messages from the left side of the window.
- From the right side of the window, click the Advanced button.
- From the drop down menu, select the proper authentication setup. Try Password, or Authenticated POP (APOP) first if you are not sure which option to choose. If you continue to have this problem, contact your ISP or mail provider for assistance.