An email server has been installed； however， the staff reports that they are unable to receive email from external email addresses， but they can email each other.  Which of the following is the MOST likely reason for this？（）

• A、 The company firewall is blocking SMTP ports.
• B、 The DHCP server is misconfigured.
• C、 The DNS server is only resolving internal name spaces.
• D、 The email server should be configured to resolve names by IP addresses.