I have 2, 5.X Unity systems. Neither one is aware of the other. One Unity is working with unified messaging, the other on a separate network in a separate domain uses Internet subscribers that will forward voice messages to the . When an outside caller calls, leaves a voice message and reaches an internet subscriber the voice message is sent to the outlook mailbox of a user on the other Unity system. When that voicemail arrives in outlook mailbox from the internet subscriber it lights the lite on the users phone.
When the email with the voicemail attached arrives from a different Unity what information in the email header tells the exchange monitor to send a MWI activation message to the Notifier queue?