how do I prevent an attacker from using email spoofing using my own email domain?

Hi there admin, i using cpanel and whm for my email, day by day, our staff often receive many spam/spoof email. i need help or maybe information how to solve this, this attacker sometimes using same name like our domain to send email to our staff, how come the attacker can using email address that not exist in our cpanel to send email to our staff. i not good on it, i have activated spam assassin but not help, now i can only block the ip address of the sender. there for maybe you can help me with this situation. many thanks for the help and effort.