MacOS mail taking too much storage space/can’t delete data

My MacBook Air has run out of storage space. I have deleted numerous files, apps, pictures, music, etc. etc. One big culprit of my storage issue is the MacOS mail app. It’s taking up 13 gigs but seemingly for no reason as I have attempted to delete all data from the app (apparently without success). I used it in the past but no longer have any use for it and just want it (and the data that’s stored somewhere) gone. I have deleted the accounts that were linked to it. I also deleted the “internet accounts” in settings in hopes that would remove the data but no luck... the app is still hogging 13 gigs. The OS won’t let me delete the app in its entirely because it’s “required by the operating system”.

why is it so hard to clear up this much needed space on my hard drive?