I have a urgent problem, I'm a business man, so email is very important to me. but I have got some problems. I have a correct configuration of my server. and I can send and recived emails, but this week some customers said that they can't send email to me, but I have tested I can recived gmail and other big company emails, just some email can't send to me, I don't know if that is my error or sender's error. recently, those email address can't send to me , such as @viapost.fr,  @outlook.com.