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