Q: mail says "this account already exists" but it's nowhere to find

You don't have to delete anything, you can but you don't have to.  There is a better solution.

1) Quit Mail app.

2) Go to System Preferences > Internet Accounts.

3) You should see your iCloud account to the left.  Under that you should see all your other email account(s) listed as "inactive".  Click the one you want to activate (it'll prompt you for your password) then to your right  there are check boxes.  Check all that you want applied, particularly the check box for "mail."

4) Enjoy.  It would not allow you to add your email account, because it was already there (hence the message saying "account already exists.") It was merely "Inactive" and needed to be reactivated.

Hi,

I had a similar problem. Thought I'd post my solution in case anyone else finds it useful. My Apple ID is also the email I couldn't get working. I kept trying to add it as a separate account, but obviously it was coming up as already existing coz it was under my iCloud account. This worked for me:

Mail > Preferences > iCloud

Under Outgoing Mail Server (SMTP) I selected Edit SMTP Server List and put in my email password (which is different to my Apple ID password) in Password (under the Advanced tab).

Thanks

Great Solution. Life-saver

Very helpful. I had tried to remove all remnants of the mail account in the Library and wasn't able to make it work.

Who'd have thought that my iCloud account would get in the way. Thanks!

Thanks for asking this question.  The response that was given WORKED!!!! 

Thanks Mate! Finally I was able to configure mail Outlook account on mail!

It worked on El Capitain 10.11.5 clean install

This worked for me as well, although the dire warnings about logging out of your iCloud account were scary.  But it worked and my mail ( and iCloud account) are back up and working well.  FYI:  this all started when I changed my password on my email account.  I went into Mail Preferences and changed the password for the pop and smtp server and it all worked just fine ... for about an hour, then it didn't work.  When I deleted the account to reset, ran into the problems described (and solved) here.  All this indicates there is some bug still to be resolved in El Capitan.

Same problem with El Capitan! Thanks for the solution.

When I went to 'Internet Accounts' I found and deleted one called 'Inactive' which was where my 'Yahoo' account should have been listed. I was then able to add the Yahoo account under 'Internet Accounts'. I closed 'Mail": I had to 'Force Quit' (Opt Cmd Esc) as Cmd clicking the Dock Mail' icon failed. When I reopened 'Mail' it fetched my eMail.

Another user happy to have found this solution. The iCloud dialogue warnings when logging off appear very end-of-the-world "This will delete all iCloud data on your computer etc...". However, all such data is recovered as soon as you reconnect to iCloud, which is the purpose of iCloud after all.

+1 for the pain this issue causes.

Thanks for the tip about removing iCloud and then adding it again.

I had exactly the same issue.  My email account (Sky) had been added before El Capitan upgrade and my email address used as my AppleID and, hence, iCloud ID.

Had to change my Sky password today (after Yahoo gave it away!!) and Mac Mail didn't seem to want to do it and my email authentication just failed.  Thought I'd remove the account and just add it in again, then got the "account already exists" problem with iCloud.

The warnings when I tried to remove iCloud from internet accounts were somewhat daunting but I persisted.  Added the email account successfully and then readded iCloud to Internet accounts.

All now seems to be well - until the next time !!