Postfix seems to be hacked

Hello everyone,

I’m having an issue with postfix on my server. It seems someone has hijacked it and it sending emails from my server. I’m getting thousands of emails like this:

This is the mail system at host pdotmedia.com. 

I'm sorry to have to inform you that your message could not 
be delivered to one or more recipients. It's attached below. 

For further assistance, please send mail to postmaster. 

If you do so, please include this problem report. You can 
delete your own text from the attached returned message. 

The mail system 

<*********@yahoo.co.uk>: host mx-eu.mail.am0.yahoodns.net[188.125.69.79] 
said: 554 5.7.9 Message not accepted for policy reasons. See 
https://help.yahoo.com/kb/postmaster/SLN7253.html (in reply to end of DATA 
command) 

Can someone please help me solve this, I’ve had to reset to shutting off postfix on the server.

Thank you