Apple Mail in El Capitan - where is data that used to be in Accounts.pilst?
I recently tested Mail in a new User account to find all the files that are modified after adding an account to Mail.
These following files contain account info. I can't say with certainty that you will need to reset all files in order to reset your account. Remember to log out/in and empty the trash before resetting. Make sure you have backup if needed.
~/Library/Accounts folder
Accounts3.sqlite Accounts3.sqlite-shm Accounts3.sqlite-wal
~/Library/Preferences/MobileMeAccounts.plist
~/Library/Containers/com.apple.mail/Data/Library/Preferences/com.apple.mail.plist