user addition problem

We have a user additional problem which is strange. We are adding users and what we find out is we can add some users and we can't add some users. All the users are getting added via email address and all the email addresses are our company email address. When i put the email address and click on invite/add button usually the user shows up in the list of users but for particularly two users even after clicking the invite/add button these users don't show up in the list neither do they get invitation email.