Mail Help
The Mail plug-in displays the number of unread mail in your mailboxes. This is done by communicating to Mail.app via AppleScript. So it will only work while Mail is running.
Query Interval

Query Interval Preferences
Set the interval in seconds at which to query Mail for unread messages and display the result.
Mailboxes

Mailbox Preferences
You may create different groups of mailboxes, each of which can be assigned a distinct appearance. In the Mailboxes pane select the group each mailbox should belong to.
Groups

Groups Preferences
Here you may add and remove groups and change the way they are displayed in the ticker.
At the moment groups will always be sorted by their name and appear in the ticker in that order.