Can someone help me with our company effort to block smart spam/phising?

Dear Network Gurus

I have a real life case as follow:

Our sales email is one of the emails that is exposed to so many people while we cannot hold the receiver security status. Some of our customer's laptop that receive our sales email is infected with virus(es). Just a total insecure device. The virus took control of the email access and stole all kinds of email addresses including ours. And now we are bombarded with spams and phising emails, but specifically for sales email.

Now if it were regular, I can just put a block on the sender address and mark as spam, but this time the hacker use our own email as the sender and the receiver. How do one face such issue and stop this spam madness?

Thank you in advance

PS: We use Email Server hosted at our hosting company and we access the settings via CPanel.