If you have enabled two step verification and use an email program like Outlook or Mac Mail, or your email program sends mail through smtp.pobox.com, you need to set up an app-specific password in order to send or receive email. (Email programs cannot use 2FA -- it is for the web only.)
App-specific passwords cannot be the same as your main account password. Once you've added the app-specific password, update your email programs to use the new password immediately.