Postfix slows because of spam
 

Hello!
I have a problem with my postfix mail server, it doesn't works properly becase of spam messages, incoming for several ones per second to the mailboxes which don't exist. And I have no idea how to reject this messages. I can't block them by ip, cause they going from different places but to the certain addresses. Looks like a virus.
Help me, please if you know what to do.