OWA Error: The Mailbox You’re Trying to Access Isn’t Currently Available

During the co-existence phase of an Exchange 2007 to 2010 migration, when you have cut over your ISA Server publishing rules for Outlook Web App to point to the Exchange 2010 Client Access server, you may encounter the following error for some OWA users.

The mailbox you’re trying to access isn’t currently available. If the problem continues, contact your helpdesk.

Depending on your Exchange environment you may find that this is only occurring for some users and not all of them. In particular you may find that Exchange 2007 mailbox users in remote AD sites are receiving the error, but Exchange 2007 mailbox users in the internet-facing AD site are not.

This can occur because of how the Client Access servers handle OWA traffic differently, depending on which AD site the mailbox user is in.

For the internet-facing AD site the scenarios are simply as follows.

Exchange 2010 Mailbox user:

1. Connects to published name for Outlook Web App
2. The internet-facing Exchange 2010 CAS connects to the mailbox server in that site on behalf of the user

Exchange 2007 Mailbox user:

1. Connects to the published name for Outlook Web App
2. The internet-facing Exchange 2010 CAS redirects them to the legacy namespace, which is published to the internet-facing Exchange 2007 CAS
3. The Exchange 2007 CAS connects to the mailbox server in that site on behalf of the user

For mailbox users in remote AD sites the situation is slightly different.

Exchange 2010 Mailbox user:

1. Connects to published name for Outlook Web App
2. The internet-facing CAS proxies the connection to an Exchange 2010 CAS in the remote AD site
3. The Exchange 2010 CAS in the remote AD site connects to the mailbox server in that site on behalf of the user

Exchange 2007 Mailbox user:

1. Connects to the published name for Outlook Web App
2. The internet-facing CAS proxies the connection to an Exchange 2007 CAS in the remote AD site
3. The Exchange 2007 CAS in the remote AD site connects to the mailbox server in that site on behalf of the user

For this to work correctly there are a few configuration requirements.

• The OWA virtual directories on the Client Access servers in the remote AD site must have Integrated Windows authentication enabled
• The OWA virtual directories on the Client Access servers in the remote AD site must not have an external URL configured (it should be blank)
• The internet-facing Exchange 2010 CAS needs a copy of the OWA resources files from any down-version Client Access servers it will be proxying to

Often people get the first two correct and leave out the third one. However without meeting that third requirement you will receive the error shown at the start of this article. On the internet-facing Exchange 2010 CAS you’ll also see an event ID 46 logged in the Application Event Log, with details similar to the following:

Log Name: Application
Source: MSExchange OWA
Date: 9/6/2011 11:42:35 PM
Event ID: 46
Task Category: Proxy
Level: Error
Keywords: Classic
User: N/A
Computer: HO-EX2010-CAHT1.exchangeserverpro.net
Description:
Client Access server “https://mail.exchangeserverpro.net/owa”, running Exchange version “14.1.323.3″, is proxying Outlook Web App traffic to Client Access server “br-ex2007-caht.exchangeserverpro.net”, which runs Exchange version “8.3.83.4″. To ensure reliable interoperability, the proxying Client Access server needs to be running a newer version of Exchange than the Client Access server it is proxying to. If the proxying Client Access server is running a newer version of Exchange than the Client Access server it is proxying to, the proxying Client Access server needs to have an Outlook Web App resource folder (for example, “<Exchange Server installation path>)\ClientAccess\owa\8.0.498.0″ that contains all the same versioned resource files as the Client Access server it is proxying to. If you will be running Outlook Web App proxying with mismatched server versions, you can manually copy this resource folder to the proxying Client Access server. After you copy this resource folder to the proxying Client Access server, you need to restart IIS before proxying will work.

To resolve this issue you simply follow the instructions in the error. Copy the OWA resource files from a down-version Client Access server over to the Exchange 2010 CAS, and then restart IIS on the Exchange 2010 CAS.

You’ll find the folder name matching the server version (eg 8.3.83.4 above) in the location where Exchange is installed, for example C:\Program Files\Microsoft\Exchange Server\ClientAccess\OWA.

After copying the files run IISReset from a command prompt.

[PS] C:\Windows\system32>iisreset

Attempting stop...
Internet services successfully stopped
Attempting start...
Internet services successfully restarted

Outlook Web App should now work successfully for all users regardless of which site their mailbox is located.