You receive an "HTTP Error 403" error message when you try to log on to your mailbox in Outlook Web Access

When you try to log on to your mailbox in Microsoft Outlook Web Access (OWA), you receive an error message that is similar to the following:

This issue occurs if one or more of the following conditions are true:

• There are front-end Microsoft Exchange computers and back-end Exchange computers in your organization. Additionally, there is a firewall or another network device that does not permit a Kerberos session to the back-end Exchange computers.
• There is a time difference among the following components:
  • The front-end Exchange computers
  • The back-end Exchange computers
  • The global catalog servers
  • The domain controllers
  By default, the permitted time difference is five minutes. Authentication fails if the time difference exceeds five minutes.

Warning Serious problems might occur if you modify the registry incorrectly by using Registry Editor or by using another method. These problems might require that you reinstall your operating system. Microsoft cannot guarantee that these problems can be solved. Modify the registry at your own risk.

To resolve this issue, synchronize the clocks among the front-end Exchange computers, the back-end Exchange computers, the global catalog servers, and the domain controllers.

If you still receive the error message that is mentioned in the "Symptoms" section, follow these steps to change the front-end-to-back-end authentication method to basic authentication:

1. Click Start, click Run, type regedit in the Open box, and then click OK.
2. Expand the following registry subkey:
   HKEY_LOCAL_MACHINE\System\CurrentControlSet\Service\MSExchangeWeb
3. Right-click MSExchangeWeb, point to New, and then click Key.
4. Type Dav to name the new key, and then press ENTER.
5. Right-click Dav, and then create the following registry entry:
   
   Registry entry name: UseBasicAuthToBE
   Value type: DWORD
   Value data: 1
6. Quit Registry Editor.

When the following conditions are true, there is an authentication issue in Exchange 2003:

• There are more than 1,000 mailbox databases in an organization.
  
  Note To work around the authentication issue, install the hotfix in the following Knowledge Base article:
  899902 Users receive a 401 error message when they try to access a mailbox that is in an Exchange Server 2003 front-end server by using Outlook Web Access
  After you install this hotfix, the new limit is 1,000 Exchange servers instead of 1,000 databases.
• Users access their mailboxes in OWA through front-end Exchange computers.

For more information about this issue, visit the following Microsoft Web sites: