I have a user who is unable to share her calendar by sending sharing invitation. The invitation goes out but says "This sharing message is not valid. All action buttons have been disabled. Then where it should say the name of the user-Calendar, it says (SenderNot Verified) - Calendar. I have tried new profile, deleting entries in credential manager, cached mode off or on, deleting cached entries on the receivers end, etc. But still the sharing messages continue.

We are on Exchange 2010, and she is using Outlook 2007 on Windows 7 with two domains in a single forest.  Recipients are a mix of Outlook 2007, 2010 and 2013.

Has anyone seen t

Just came across this post while perusing for an answer to the exact same issue.  The user has Outlook 2013 and has successfully sent a calendar sharing invite to several users with a mix of Outlook 2013 & 2010.  However, there are a few users who receive the (Sender not verified), with the option grayed out to "Open this Calendar."  Tried same troubleshooting methods of creating a new profile, turning cached mode off, deleting/recreating offline address book; same result.