Yutaka-Notes about Extensible Classes In ZimbraStore
By this, you can register your custom function to be called when a messege is receive with lmtp.
By this, you can implement to genearate custom auth token and to validate that.
Sample is here; com.zimbra.cs.service.ZimbraAuthProvider
[Note] You have to register this to com.zimbra.cs.service.AuthProvider