Do you get an equivalent message if you send a test message using another email system such as Google Mail?  If so, it is most likely an issue on the recipient's gateway.  

Hi Ben, we tried, but not working.

Hi Jay Marme,

Thanks for suggestion. Yes we tried but got same issue.

We add users account in Domino server, and change his/her email address to personal email, such as Gmail.

I'm not sure such steps is correct or not.

Alvin.