There are many solved problems in open source. Groupware is not one of them.
How else would you explain the number of migrations that fail on average ingroupware? The Swiss canton of Solothurn is just one example among many as a result of groupware vendors who have given up and transitioned to Outlook or the web to meet their needs. Kolab does things differently. For one, Outlook will never be the client for the Linux desktop. And, the web is a good answer for a lot of things, but not all.