let me ask the question in another words, I want to "Restrict incoming mails (certain domains) to be received by a certain groups and restrict the outgoing mails to be sent to certain domains for specific groups either." is this applicable in smartermail? 

Hello.  Content Filtering is only applied to inbound messages.

Good afternoon. This question has been asked 5 or 6 times in this forum, but I don't believe it has ever been fully answered with a working solution. We just did some testing on this and it looks like there may not be a way to do what you are asking in Smartermail. However, you can use Declude content filters to accomplish this. If you don't already know, Declude is free anti-spam software that we offer on our website. You can download it here http://mailsbestfriend.com/downloads. You don't have to use it for spam filtering. You can simply configure it with the specific filters that you need to use without affecting anything else on your server. I hope this helps.