Changed Office365 password, cannot change in mail.app: account already exists mac os 10.11.4
mail app 9.3


Had a working exchange/office365 account. Did not work anymore, used safari to go to office365.com and it appeared that I was required to change my password. So I changed it and can login on www.office365.com
So the mail.app tells me to change the password, but when I hit save I get: account already exists.

So I removed the account from the settings --> internet accounts. closed mail.app, rebooted the mac and tried to add a new exchange account. But again "account already exists"

This seems similar to Mac Mail - Account already exists error when adding gmail
but iCloud is on a totally different mail account and even a different domain. 
Before I removed the exchange account, I manually updated all office365 passwords in the keychain. But that did not work either. Mail.app kept complaining. 

Please do not suggest to move to outlook. I have outlook installed, but my boss (owner of the macbook) want to stick to the default mail.app
I had the same thing happen to me this evening after updating my Office365 password. 

What worked for me was to open my desktop Outlook App and send myself a test email. Outlook was then prompted to ask me to put in the password. After adding the password to Outlook and sending the test email, I then went back to my Mac Mail and received my test email there. I also checked the Outbox in Mac Mail, which had three emails waiting to send. I double-clicked on the first one and then all three emails sent.