Hallo,
unfortunately I a have a complex scenario of servers an clients and some problems with syncing calendar invites with iOS and the other way around.
My company is using Exchange 2010 as their e-mail server. The clients are still running Outlook 2007. I am not responsible for these clients and servers and so I cannot change anything on that 
The users are creating calendar invites on Outlook 2007 and sending these invites to my zimbra 8.0.3 e-mail server through the internet. The iPhones with iOS 6.1.3 are connected through exchange with zextras mobile 1.8.5.
When a user creates an invite on Outlook 2007 the iPhone user gets this invite and can accept or decline it. The outlook user gets the answer as an email with a "not supported format.ics" as attachment. If the Outlook user does not open the attachment the accept or decline will not be added to the calendar. If the Outlook user opens the attachment the accpet/decline will be added. This needs just some extra clicks so it is not such a big problem. Nevertheless any ideas would be appreciated 
My problem is, if the outlook user modifies an existing calendar entry and sends this to the iPhone users, the iPhone users get this change, they can accept/decline. The Outlook user gets the accept/decline as an email with attachment but only gets the message that the user accepted/declined this invite before the calendar entry was changed and unfortunately the accept/decline will not be added to the calendar of the Outlook user.
I am not sure where the "problem" is because there are so many different systems with outlook 2007, exchange 2010, zimbra and zextras mobile with EAS 2.5 so I am asking here. 
Thank you for your help!
Alex