When you try to connect to Outlook Web Access, Remote Web Workplace, Companyweb, or any similar web applications, the site fails to load, and you receive the following error message:

Generally, this error occurs if the application pool that is associated with the web application doesn't start. To troubleshoot this issue, follow these steps:

1. In Event Viewer, view the System log to find errors from the Microsoft-Windows-WAS source. An event is frequently logged in the System log if the application pool fails to start.
2. If you don't find any relevant events in the System log, search for relevant entries in the HTTPERR log file. The httperr1.log file is located in the following system folder:
   c:\windows\system32\logfiles
   In the file, search on "503" to locate any relevant information about why the application pool failed. For example, you may see an entry that resembles the following:
   2011-12-08 18:26:42 ::1%0 6721 ::1%0 443 HTTP/1.1 GET /owa 503 3 N/A SharePoint+Web+Services+Root
   The log entry should list the site you are trying to browse and the application pool that is being used. Make sure that the correct application pool is being used for the site.

For example, you try to browse the "/owa" website by using the Sharepoint Web Services Root application pool. The OWA application is supposed to use the MSExchangeOWAAppPool application pool. However, there is a misconfiguration in Internet Information Services (IIS). To resolve this issue, you would check the Application Pool for the site and for the site bindings to make sure that IIS is configured correctly.

1. Open IIS Manager MMC. To start IIS Manager from the Administrative Services console, follow these steps:
   1. On the Start menu, click All Programs, click Accessories, and then click Run.
   2. In the Run text box, type control panel, and then click OK.
   3. In the Control Panel window, click Classic View, and then double-click Administrative Tools.
   4. In the Administrative Tools window, double-click Internet Information Services.
2. Check the bindings for each of the websites to make sure that they match the default settings in the following table.

   Website	Type	Host Name	Port	IP Address	Binding Information
   Companyweb	http	Companyweb	80	*	Port 80
   Default Website	HTTP http net.tcp net.pipe net.msmq msmq.formatname https https		80 80 443 443	* 127.0.0.1 * 127.0.0.1	808:* * Localhost localhost
   SBS Client Deployment Applications	http http	SBSConectComputer	80	*	*:80:connect
   SBS SharePoint	https		987	*
   SharePoint Central Administration v4	http		18000

3. Check each Application Pool value to make sure that the settings are correct. The following table contains the default settings for the standard SBS application pools.

   Application Pool	.NET Framework Version	Identity	Associated Applications
   DefaultAppPool	V2.0	ApplicationPoolIdentity	/rpc /rpcwithcert
   MSExchangeAutodiscoverAppPool	V2.0	LocalSystem	/Autodiscover /Autodiscover/bin /Autodiscover/help
   MSExchangeECPAppPool	V2.0	LocalSystem	/ecp
   MSExchangeOWAAppPool	V2.0	LocalSystem	/owa
   MSExchangeOWACalendarAppPool	V2.0	LocalSystem	/owa/Calendar
   MSExchangePowerShellAppPool	V2.0	LocalSystem	/PowerShell
   MSExchangeSyncAppPool	V2.0	LocalSystem	/Microsoft-Server-ActiveSync
   MSExchangeServicesAppPool	V2.0	LocalSystem	/EWS /EWS/bin
   SBS Sharepoint AppPool	V2.0	<Domainname>\spwebapp	Root Application for SBS SharePoint Root Application for Companyweb
   SBS Web Applications application pool	V4.o	NetworkService	/webhelp Root Application for Default Website
   SBS Web Workplace AppPool	V4.0	NetworkService	/Remote

The following documented issues commonly cause an HTTP 503 "service unavailable" error.

Issue 1: SharePoint AppPool identity is invalid
Issue 2: Access denied on Exchange SQM files
Issue 3: Application pool is enabled for 32-bit applications