Suppose my domain is abc.com. I got a distribution group (ABC-Everyone) including all users in that domain and I only want to allow certain people send email to this group. I want to allow all users in abc.com to send to this list, so I add ABC-Everyone in the allowed list. If I also want to allow all users under the @123.com (external domain) to send to this list, Is that right that I first required to add all users in 123.com as mail-enabled contact, then create a distribution group for them and add this group to the allowed list?

Hi,Yes, you need to create contacts for 123.com domain and add them into DG for allowed list of users.If you are using Exchange 2007, you can use Transport Rules to achieve this with various Predicates.Understanding How Transport Rules Are Applied in an Exchange 2007 Organizationhttp://technet.microsoft.com/en-us/library/bb124703.aspxAmit Tank | MVP - Exchange | MCITP:EMA MCSA:M | http://ExchangeShare.WordPress.com