If the Use current user name and password check box is selected, Microsoft Exchange will use the credentials of the Microsoft Exchange service account specified during Setup. These are the credentials used by the message transfer agent (MTA), and are therefore the only default credentials available.
If the local service account has full Administrator permissions in the remote Microsoft Exchange site, this method will work. However, the Microsoft Exchange MTA requires a full-duplex communication link between the two systems. This means that the remote MTA will attempt to bind back to the local computer. To accomplish this, the credentials from the remote system must also be present and validated to access the local system.