When sending a meeting invite to a affected user we received a non-deliverable instantly.
Blog site: http://blogs.platani.nl/?p=529
The non-deliverable message was:
#554 5.2.0 STOREDRV.Deliver.Exception:DelegateUserValidationException; Failed to process message due to a permanent exception with message Delegate user validation failed for user: username. DelegateUserValidationException: Delegate user validation failed for user: username. ##
After a lot of searching without much result, I figured out that it might have to do something with the free busy information.It appeared to be corrupt free busy information in the user mailbox. The procedure to reset this is outlined in KB article 958443(below)
Required steps
To be complete I also inserted the required steps below:
•Exit Outlook.
•Make sure that there is an online mode profile for the mailbox.
Note If there is no online mode profile, use the Mail item in Control Panel to create an online mode profile for the mailbox.
•Download the Microsoft Exchange Server Messaging API (MAPI) Editor tool (MfcMapi.exe). To do this, visit the following Microsoft Web site:
•Start the MfcMapi.exe program that you downloaded, and then click OK.
•On the Session menu, click Logon and Display Store Table.
•In the Profile Name list, select the online mode profile for the mailbox, and then click OK.
•Double-click the Mailbox – username store.
•In the navigation pane, click Root Container.
•In the details pane, right-click PR_FREEBUSY_ENTRYIDS, click Delete Property, and then click OK.
•In the navigation pane, expand Root Container, expand Top of Information Store, and then click Inbox.
•In the details pane, right-click PR_FREEBUSY_ENTRYIDS, click Delete Property, and then click OK.
•In the Information Store: Inbox dialog box, click Exit on the File menu.
•Exit the Microsoft Exchange Server MAPI Editor.
•Click Start, click Run, type outlook.exe /cleanfreebusy, and then press ENTER.
Ex-Shell 