If you assign a domain controller the Global Catalog role, and take the existing Global Catalog offline for maintenance, your Exchange Server stops providing email?
If you check the event log, you notice multiple Event 9167 errors:
Event Type: Error Event Source: MSExchangeSA Event Category: NSPI Proxy Event ID: 9176 Date: 5/29/2001 Time: 10:03:04 AM User: N/A Computer: <YourExchangeServer> Description: NSPI Proxy can contact Global Catalog <YourNewGlobalCatalogServer> 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 <YourNewGlobalCatalogServer> as soon as possible.After you restart <YourNewGlobalCatalogServer>, Exchange starts providing email.
taking the existing Global Catalog offline, restart the new Global Catalog.