I had the same problem. You are supposed to delete people from being delegates BEFORE you delete their AD account, but sometimes that doesn't happen.
In my case, I knew which user caused the NDR to be sent. We checked that they had no rules in their mailbox (export them if they do have rules), then started outlook with the command line switch /cleanrules. That fixed our problem.
You can also write a script to return all accounts with delegates, but I don't recall how. You search for delegatesBL and publicdelegates in AD, if I remember correctly. I'll see if I can find the script.
Here is the reply from MS when I opened a similar case:
Issue
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Members are getting NDR from One user whose AD account was deleted
Resolution
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This user must have been a delegate to many users and was not removed from managers’ Outlook before her AD account was disabled. Here are steps we tried :
1. On the Tools menu, click Options.
2. Click the Delegates tab.
3. Click the delegate that you want to remove, and then click Remove.
4. Click OK to close the Options dialog box.
Note After the delegate has been removed from Outlook 2003, the delegate's rule is removed. Then, the delegate can be removed from Active Directory.
>>But we could not find her listed as a delegate, therefore applied the steps below :
1. Checked if the user had any rules, none present
2. Ran Outlook /cleanrules
We were then able to send meetings to only the manager without getting any NDR from the delegate and no emails were sent to the delegate’s mailbox.
The detailed information of this behavior is given in the Knowledge Bas article :
Members of your organization may receive a non-delivery report if your delegate’s mailbox object is removed from Active Directory before you can remove the delegate in Outlook 2003
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MCSA: Messaging 2003