Many of the Synthetic Transactions (ST) in Lync Server 2010 allow you to specify a user and the corresponding user credentials, but do you always have to specify the user credentials? The answer depends on from where you run the ST.
If you run the ST from a server, which is trusted by the Lync server you run the ST against, you don’t have to specify user credentials. The trust is established by defining the server in the Lync topology, install Lync on the server and assign a certificate to it. The server running the ST will then use MTLS when communicating with the target Lync server.
If you run the ST from a server with only the Lync Admin tools installed on it you need to specify user credentials. It is necessary because otherwise it is not possible for the ST to authenticate against the Lync server it is targeting, since the server running the ST is not trusted. If you forget to specify user credentials you might see an error message like A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider:SQL Network Interfaces, error: 26 - Error Locating Server/Instance Specified).
Thanks to Antenehe and Fatih for background information.