But I like the idea of entering the password in Internet accounts instead just for good measure since that's where the calendar, contacts, etc. ***For step 2, I alternatively did test re-entering the password in Apple Mail directly in the password field in Preferences->Accounts->the email account->Server Settings and that also seemed to work just fine. Click the error/exclamation or go to the top menu->Mailbox->Take All Accounts Online and back in business! The account now appears but there's a new error / exclamation point appearing, but it's simply that the account is offline. The status circle should light up green and say connected, but the account still might not be showing in Mail (e.g. Reopen Apple Mail, and re-check 'enable this account' in Apple Mail. Go into System Preferences->Internet Accounts->the email account-> Enter the new password here In Apple Mail, go into Preferences->Accounts->uncheck enable this account.

Here are the steps that worked to fix this: The user was specifically running Mac OS 10.14, though this fix may work just as well for other Mac OS too. 'Can't connect to the account.Enter the password for user.Unable to verify account name or password.' It should not be confused with same-sign. It recognized that the password had changed but wasn't accepting the new password, even though we knew the password was correct. True single sign-on allows the user to log in once and access services without re-entering authentication factors. Following a password reset (specifically a Microsoft O365 account), Apple Mail was not happy.