The recipient or groups FYDIBOHF23SPDLT might be removed from the organization.
Or we've used the Outlook cache address to replied the meeting request. Please try to clean it from File > Options > Mail > Send messages tab > Empty Auto-Complete List.
Or we may create a new Outlook profile to check if we still get the NDR in a new profile.
http://support.microsoft.com/kb/829918
Cheers,
Tony Chen
Forum Support
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