We've come across a strange error with Outlook calendar sharing. We have a particular user who has created a custom calendar and sent out several calendar sharing invitations. Several users received the invitation and were able to open the calendar, while a few others, including myself, were not able to open the calendar. The invitation contained the following error: "This sharing message is not valid. All action buttons have been disabled." (Sender not verified). For the user sending the invitation, I've tried disabling cache, deleting/recreating offline files folder, updating the address book and finally creating a completely new Outlook profile. I verified in AD that the users email address appears within general properties tab, as that was a suggested resolution on another forum. Also performed the same troubleshooting actions on one of the users who was not able to open the calendar.
We are running Exchange 2007 with a mix of Outlook 2010 & 2013. All users are running Windows 7 Pro and there are no POP3/IMAP4 accounts.