Changed default mail client, but Spotlight still opens Mail.App; how to change it?
The best way I've found to set any default app is with RCDefaultApp - freeware - old but still works in Yosemite.

It installs as a Control Panel item called Default Apps; you can then set many defaults - simplest are the 'Internet' ones, first tab - dial in your preferred app from the drop menu.
e.g.