Can not edit Calendar Items on iPhone 4 that were sent from Outlook 2013

Hi Folks,

When receiving a calendar item from Outlook 2013, I can accept it, but not edit it. When I receive the same item from Outlook 2003, I can accept AND edit.

How can I edit calendar items sent from Outlook 2013?

We are not using an Exchange server, simply sending the calendar items via email. We noticed this when we upgraded our office from MS Office 2003 to 2013.

Thank you so much for your help!

Ben Bolduc