Error

Microsoft.ServiceModel.Channels.Common.ConnectionException: Login failed. The login is from an untrusted domain and cannot be used with Windows authentication.
—> System.Data.SqlClient.SqlException: Login failed. The login is from an untrusted domain and cannot be used with Windows authentication.

The user was correct, and that was driving me crazy. I had just built the database from a script, and I set and double-checked the security in SSMS for that database.
I’m running on my own VM on my desktop; and the userid is my VM-Name/bts_host_ins (and that’s the user that all the host instances run with).

Solution

There could be other reasons for this error, but here was my reason…

My SendPort was pointing to our development system which is on the “DEV” domain, and my VM where I’m testing is on the primary domain.
The fix was just to repoint the SendPort to my local VM which is what I wanted anyway. If I really did want to go to the DEV domain, then I would have to use a user on that domain to run my host instance.