This seems to be a common issue here with this new upgrade for Mac users whereby clicking on a contact's email address no longer opens up their email client.
Here is some information from Apple and other forums about issues with Yosemite and sending emails:
http://osxdaily.com/2014/10/28/fix-mail-smtp-sending-errors-os-x-mac/
https://support.apple.com/kb/PH19184?locale=en_US
https://discussions.apple.com/thread/6609827
We would suggest that you contact Apple on this by phoning their Apple Support Line in the UK on 0800 048 0408