I have created dynamic distribution group and allowed only authenticated users to sent mail. One of our user connecting to exchange using POP3 settings with authentication cannot sent mail to this group. He is getting the error message “Your message wasn't delivered because of security policies. Microsoft Exchange will not try to redeliver this message for you. Please provide the following diagnostic text to your system administrator”. We unchecked the authentication tab from dynamic group settings and he is able to send messages. But we would like to activate this authentication option and need to allow internal pop3 users with authentication settings.