The Global Mail Rules is for the administrator to further refine their email traffic. It is applicable on the incoming email sender's email address.
You have to login as Avomaster/voadmin in order manage these mail rules.
Go to Profile → Admin Task → Global Email Rules to access the configuration.

Rules syntax:
. Each line is a rule. A rule may contains wild card like those used by the Bourne shell.
. Each rule is prefix with an action, like this: [action] [space] [rule] [cr]

Examples:
The following will allow all email from yahoo.com to get pass:

 pass *@yahoo.com

Note that a “*” sign if very important here.

The following will mark all emails from somedomain.com as Junk:

 junk *@somedomain.com
 

The following will reject (stop delivery) of all email coming from hotmail domain.

 reject *@hotmail.*

Note that the * at the end will basically block all hotmail domain including:
hotmail.com, hotmail.net or hotmail.cc

The following will allow one person vip@domain.com to pass the rule:

 pass vip@domain.com
 junk *@domain.com

This would basically block all other email from domain.com other than vip@domain.com.
However, pay attention that the pass rule must place before the junk (or reject) rule, or else it will not work this way.

Action tags available:
pass Let the email pass. The Junk ranking will set to 0.
reject (or block) Reject the email, not going into the delivery system.
junk Deliver the email, with Junk rank set as 5 at least.