Is the value in "Computer" and "Source Workstation" the same server?
If not, are they in the same domain?

If the two machines are in different domains, then this is likely what
is happening:

When DTC negotiates a connection with another transaction manager, it
will always attempt a secure connection first. If this fails, and
security is disabled for DTC, then it will attempt a connection
without passing any credentials.

In the above scenario the two servers enlisting in a transaction were
in seperate domains. When DTC would try to connect with DTC on the
remote server, it did so in a secure manner and passed the
machine account. This account did not exist in the domain of the
remote server, which resulted in the failure audits in the security
logs with an HRESULT of 0xC00000064. This error code means no such
user exists. DTC then attempted an unauthenticated connection that was
successful and allowed the application to work.

A Netmon trace should show you this behavior.

Hope this helps.

Steve Antonio, CISSP
Microsoft Exchange Support