I am struggling with the same issue raised here - "Unable to verify account name or password" on outlook.com Exchange server (browser works). It started after I updated MacOS to Mojave this week. For me everything worked fine (even after upgrading the OS) until the first restart. At that point Mail started complaining it could not verify my credentials.
Here's what I've tried so far:
Confirmed my credentials still work with my @hotmail.com/Office 365 account.
Created a new user on my MacBook Pro and tried to connect to the account as an Exchange account.
Reset my keychain.
Changed my Office 365 password.
Tried every esoteric solution reported on the internet that I could find.
I was able to connect Mail to my hotmail account by adding it in as an IMAP account. However, this only works for Mail and not for the calendar and contacts.
Any help in trying to find a solution would be greatly appreciated.