电子邮件不会进入私人域名

I am trying to send an email to all the staff members once particular action triggers. Everything works fine. However, I have noticed that email is not going to those who have our domain name email address. Other than that, I have tried to check that the one who use gmail, yahoo or hotmail receives the email. Most interesting thing is that, they can see the list of all email addresses including those who do not receive it. I am just wondering, what could be the reason for it ? Is there any restriction for sending any emails to private domains ? I do not reckon. I am using the I am trying to send an email to all the staff m