Outlook 2003 connectivity issues with Exchange 2010
Although this error message has multiple causes one I have seen recently is due to Exchange 2010’s Default Throttling Policy with Outlook 2003. You may notice this error occur when opening additional shared mailboxes or calendars from Outlook.
The connection to the Microsoft Exchange is unavailable. Outlook must be online or connected to complete this action.
Using this command you can see the number of current RPC connections for a user. By default Exchange 2010 Throttling Policy allows 20 RPC connections per user so if you start receving this error once you hit 20 connections you know the Throttling Policy is causing your problems.
Get-LogonStatistics -Identity “username” | fl applicationid
The current resolution options include creating a less restrictive throttling policy and applying it to the effected users. There are reports that Outlook 2003 establishes more RPC connections than Outlook 2007/2010 as they are more efficient with their connections.
New-ThrottlingPolicy –name NewPolicy
Set-ThrottlingPolicy –identity NewPolicy -RCAMaxConcurrency 100
(I have used 100 here but any figure above 20, the default, may assist you depending on how many connections the client is establishing)
Set-Mailbox –Identity “user” –ThrottlingPolicy NewPolicy
It’s important to restart the throttling and rpcclientaccess services once you have applied to new policy as without this the changes won’t apply.
Thanks to the follow blogs for helping to point the way on making throttling policy changes apply.
In relation to working with Exchange 2010 and Outlook 2003 clients this is the article to read.