Single Single-On with NT Domain and Exchange 5.5?

I’ve had a few associates state that they have single sign-on capabilites in an NT domain structure with Exchange 5.5. This means, once authentication has occured into the domain (through Windows login after the machine boots up) – they do not have to provide their username and password again when opening Outlook to access their Exchange 5.5 mailbox.

I’m perplexed at this – considering there is no kerberos involved here, I don’t see how this is possible. Can anyone shed any light on this topic?