Unfortunately Apple love forcing their software on users and providing no way to uninstall. If you’re sick of apple mail popping up every time you click an email link or pressing cmd + shift + i (as I often do trying to get into the web dev tools in chrome) do the following in your terminal to remove it:
sudo -i mkdir /dump mv /Applications/Mail.app /dump mv /usr/bin/mail /dump chmod 000 /dump
You could also rm -rf the files but I like to keep them around just in case OSX breaks somehow. If you don’t run the chmod command OSX will actually detect the file has moved and ask you to set it up again.