Apple Mail in El Capitan - where is data that used to be in Accounts.pilst? I run about six IMAP accounts on two different OS X machines and one iOS device. 

Several of the mail accounts are under different domains, but all hosted on the same 3rd party mail provider so presumably running identical IMAP and SMTP servers. Also, the settings on the OS X mail clients in Mail/preferences/accounts look identical, except for the account-specific settings like name/password, etc.

Nonetheless, still I have a several undesired behaviors, different both between accounts and between clients, which I can't seem to fix. 

Prior to El Capitan I would have looked for causes of these differing mail account behaviors in ~/Library/Mail/V2/MailData/Accounts.plist. That file is still there, but it seems to be an unused, outdated leftover from an OS upgrade, as it shows no current mail account settings.

Meanwhile, looking at the several plist files in both ~/Library/Preferences/com.apple.mail and ~/Library/Mail/V3, I find none of the mail account settings I want to see.

So in El Cap, where does one find the equivalent mail account settings data that used to be in ~/Library/Mail/V2/MailData/Accounts.plist ?

(OSX 10.11.3, Mail 9.2)
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