Outlook error – The Delegates settings were not saved correctly

I have given some users permission to access a shared mailbox in Exchange, however when updating permissions on the Delegate Access page in Outlook the following error message was presented:

The Delegates settings were not saved correctly.  Cannot activate send-on-behalf-of list.
The operation failed.

The solution was quite simple: there was a user listed that didn’t exist in AD any more, so once I removed them I was able to continue and add the new users.