The distinction between search.yahoo.com and mail.yahoo.com is important (among many others).
I'd like a Referring Subdomains report, similar to what others tools have.
This is a good one. I'm going to explore it with our engineering team. Thanks for submitting and voting. Please continue to vote and comment on it!
One tip/workaround: With Processing Rules, you can get to something like this pretty easily, with no changes to JS code/on-page implementation. Just set up a rule where, on every hit, you are copying Page Domain (which includes subdomain) into an available eVar.