This was a nice little problem, and a neat fix, so I thought I'd share it.
I got a call from a customer who was having a small problem with a senior manager, which always seems to be the way!
The managers PA had left, and the user object and Mailbox had been deleted and purged.
Now, everytime anybody booked a meeting/sent a meeting request to Mr Manager, the sender got an NDA from the missing PA!
The error was " The e-mail account does not exist at the organization this message was sent to. " which makes sense, with an SMTP status code 5.1.1
5.1.1 is a fairly new code, I won't break them all out here, but 5.x.y is a fatal error, and the first 1 is an information report. 5.1.1 basically is informing you that the mail didn't get thru, because the user doesn't exist, which is no real surpirse.
The reason this happens is pretty simple.
When you set up delegate access to a calendar using the Delegate Access Wizard in Outlook, you have options (depending on which version of Outlook) to have meeting requests sent to your delegate.
The wizard sets up a hidden rule within the mailbox.
If the delegate is deleted before the delegate permissions are removed, then the rule is orphaned.
There is a download detailed in PSS ID: 253557 http://support.microsoft.com/kb/253557/en-us that walks you thru the MDBView utility, which can be fun, but try this first.
Log in via Outlook Web Acces.
Click the Rules button.
There is a rule listed with <no name>
Delete it.
Problem solved!