Just yesterday I got a call from a colleague about his inability to connect to a SQL Server 2016 instance from the application interface. The error being returned was “Test failed. Login failed. The login is from an untrusted domain and cannot be used with Windows Authentication”
In my past experience this error would happen under one of the following scenarios:
The client machine is not joined to the same Active Directory Domain as the database server
The user is trying to use an SQL Login for an instance configured for Windows Auth ONLY
There is a mismatch between the the supported NTLM version on the client and the database computers. (See Server Manager > Tools Local Security Policy > Security Options > Network Security: LAN Manager Authentication Level)
In this particular case everything seemed OK. The problem simply went away when the password for the account being used to connect was reset. Any ideas on the cause are welcome.