Difference between "Message" and "MUC Private Message"

Hi,

I am trying to set up a packet filter to prohibit specific users to communicate privately, except with the management.

Firstly, I set up a Reject rule on all MUC Private Messages from “any” to “any”

Secondly, I set up a Reject rule on regular messages where from/to users are both in the group of restricted users.

However, It looks like all private messages are rejected by the first rule. How can this be? What is the difference between these two message groups, if not to distinguish between regular private chats and private chats via a MUC (conference) room? Have I misunderstood something?

What client are you using?