All Global Catalog Servers in user are not responding:

Environment
(1) Exchange Server running Server 2003 and Exchange 200
(1) Domain Controller running Server 200
Clients running XP and W2
Email clients running Outlook 200
All systems are current on patches

From time to time clients lose connectivity to the Exchange server.

A review of the Application Log in Event Viewer shows
Event Type: Erro
Event Source: MSExchangeDSAcces
Event Category: Topology
Event ID: 210
Date: 4/30/200
Time: 5:43:19 A
User: N/
Computer: <Exchange server name
Description: Process MAD.EXE (PID=2364). All Global Catalog Servers in use are not responding: <Domain Controller Computer Name>.<Domain>.local

A subsequent entry sometimes follows the above

Event Type: Erro
Event Source: MSExchangeS
Event Category: NSPI Proxy
Event ID: 917
Date: 4/30/200
Time: 7:11:43 A
User: N/
Computer: <Exchange server name
Description: NSPI Proxy can contact Global Catalog <Domain Controller>.<Domain>.local but it does not support the NSPI
service. After a Domain Controller is promoted to a Global Catalog, the Global Catalog must be rebooted to support
MAPI Clients. Reboot <Domain Controller>.<Domain>.local as soon as possible.

A reboot of the domain controller is not necessary as the problem typically "cures itself" within about 30 minutes.