Sending to group problem

Tile of question : problem when Sending message to groups in exchange 2000

My environment consist of Active directory with tow domains , Ad and WB , Exchange Server is joined to WB domain.
*All domain controllers are in the same site .
* FSMO and global catalog servers are distributed between domain controllers.
* Exchange 2000 Version 6.0 (Build 6249.4: Service Pack 3) Enterprise Edition installed on Win2K Advanced Server with SP4.

* Problem description:
When I send message to a local group,some times this message doesn?t reach the recipients ( members of this group),and the message appears in sent items folder .

When I track the message I notice the following

8/7/2005 12:05 PM SMTP: Message Submitted From Store .
8/7/2005 12:05 PM SMTP: Message Submitted to advanced Queuing .
8/7/2005 12:05 PM SMTP: Started Message Submition to Advanced Queue.
8/7/2005 12:05 PM SMTP: Message Submitted to categorizer .

Note : the message does not delivered locally to multiple recipients

if any one h