Postfix milter problem

[ArtemF]

Hello,
I've encountered a problem:

Lots of incoming emails are rejected with error:

milter-reject: DATA from somehost.tld[xxx.xxx.xxx.xxx]: 451 4.7.1 Service unavailable - try again later;

Restarting pc-remote and postfix as well as increasing milter timeouts and adding "milter_default_action = accept" in main.cf doesn't solve a problem.
Errors stop if greylisting is turned OFF for entire server.
When greylisting is turned ON globally, problem affects even domains, that have greylisting turned OFF in domain mail settings(i checked it with "grey_listing --info-domain").
Server is running plesk 11.5.30 MU28 and is not overloaded.