Calendar events sent to Outlook client do not work

Greetings,

I've come across an odd behavior with Kolab calendar ICS event that are 
sent to users using Outlook as their email client: events just show up 
as a downloadable ICS attachment instead of giving them the option to 
Accept/Deny/Maybe like other events. I've confirmed this on Outlook 
2003/2007/2010/2013 and Office 365 webmail. Testing it with Gmail 
accounts worked without issue.

The only thing I can think of is that Outlook doesn't like the ICS 
formating. Is this because Microsoft is again not conforming to 
standards? I hope there's a workaround, a lot of people use Outlook in 
my part of the world.

-- 
Mat