Exchange 2013 add "SPAM" at the beginning of message subject

Hello!

I have ordinary Exchange 2013 CU3. Anti-spam agents are not installed. In front of Exchange I use anti-spam server, that marks SPAM messages with X-SPAM headers. Exchange 2013 modifies message subject and adds "SPAM" at the beginning of it (I do not know why.)

How to disable this feature? I do not wont message subject being modified.

Any help would be appreciated.

Thank you.

Pavel.