When using /authuser with Exchange I get the error 'Can't do /authuser with this server.'
As you surmised, Exchange does not support it.
I then set up a local test CommuniGate Pro server to try it.
Your test shows that Communigate Pro doesn't support it either; or at least you attempted to use an id that did not have administrative privileges.
These are server issues. If it isn't supported in the server, there is no way that a client can offer the facility. At least, unlike PROXYAUTH, you can ask the server vendors to support it and point to an RFC which describes it.
-- Mark --
http://staff.washington.edu/mrc Science does not emerge from voting, party politics, or public debate. Si vis pacem, para bellum.
