A few years ago I used to own a website which was forever having it’s email spoofed. How ever I only found out about the problem after someone emailed me complaining. Naturally I rushed to find a solution to the problem. Below is the solution I found:

SPF (Sender Policy Framework)
Essentially this is an insanely technologically confusing method, but to put it simply…Only the server you specify can send email from your domains. It’s useful when it comes to the bigger mail services out their as they like to confirm that the email’s source is correct.
If you are using cPanel you can enable SPF by going to the “Email Authentication” page and clicking enable.