Mail app is using 10GB on my laptop (High Sierra 10.13.3). I don't use it at all; I usually use the web interface and occasionally Thunderbird for my Gmail account.
This is the only Mail-connected account in Internet Accounts. If I were to uncheck it in Internet Accounts for my Gmail account so that I can delete the cache and free up some space, is there the possibility of any unintended consequences? For example, will I somehow stop getting email reminders of things in my BusyCal calendar? Or some other possibility that I am unaware of?