Generally you can use any greylisting solution that works with postfix beacuse ISPConfig does not configure the postfix main.cf and master.cf files. One well known postfix greylisting server is postgrey, but I have no experience with it yet.
I have installed postgrey on my ISPConfig-managed server. It works very well, even though occasionally you'll have to wait for a few minutes before a new mail is received - some servers do not honour the banning time and only resend the message after 10 minutes or so. ISPConfig does not interact with postgrey so no special instructions are needed. If you are a newbie, just install postgrey and leave it like that. It will work by itself. Otherwise, customize the error message and banning time and add abuse@ and postmaster@ to the server-wide whitelist. All of this is done via conf files in postgrey dir /etc/postgrey.
OK, i´ve just done the changes in this post, will this work without any mor changes? http://www.howtoforge.com/greylisting_postfix_postgrey
