Error "Cannot open Mailbox" when attempting to open an Exchange 2007 mailbox from Exchange 2010

The scenario is that you are logged into a Exchange 2010 mailbox and want to access a shared calendar or mailbox on Exchange 2007.   When attempting to open the mailbox, you may receive the following error/ (full call stack below): Request Url:https://server.company.com:443/owa/proxyLogon.owa   Exception Exception type:Microsoft.Exchange.Data.Storage.StoragePermanentException Exception message: Cannot open mailbox /o=Company/ou=Administrative Group/cn=Recipients/cn=testuser1.  …

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