1. The Microsoft Exchange MTA Stacks service cannot start on a Microsoft Exchange Server 2003 computer, and this event, along with some few more events, may appear in the application event log.
2. The MTA Stacks service may not start on computers that are running Microsoft Exchange 2000 Server Service Pack 2 (SP2) or later. Microsoft Outlook clients that connect to the Exchange 2000 server may have outgoing e-mail messages that sit in the Outbox and cannot be sent.
3. The Microsoft Exchange Server Message Transfer Agent (MTA) may fail to start.
Following links to Microsoft article give more information about this event.