There are plenty of answers to the error message:
"Delivery has failed to these recipients or distribution lists:
Your message wasn't delivered because of security policies..."
In our case, Jane Doe was no longer an employee and her account was disabled. The sender of the message was requesting a meeting with someone who had Jane as his admin, so he had her as a Delegate for his Calendar. Once we removed her from his Outlook's Delegate list, there were no more errors.
Pretty simple, but they've moved Delegate Access in Outlook 2010, now you go to File > Account Settings > Delegate Access
For Outlook 2007 and earlier, go to Tools > Options > Delegates tab.