Your question is a little confusing so I will answer in the best way I know.
In UC 7.x the system works in the following way:
When you use message notification an email is sent from the unity account on the UC server to the user. If you reply to this message the reply will be sent back to the UC system but not to a UC subscriber.
Unity does not place the email address of the other user in the message so there will be no way to directly reply to the users exchange inbox. Nor will the message go back to the user who sent it because the message notification is sent from a UC system account and not from the user. The message notification is a one way message sent out from UC to the user to notify them of a new message in the UC inbox. With that message you can only reply to the Unity system account.
Please let me know if this answers your question.
I think you understood well my question. After posting our Cisco SE contacted the UC Product Mgr and here was their response (assumption was UC 8.5).
“Can a Unity Connection 8.5 single-inbox user replying to a VM user’s voice mail with an email?” The answer is yes. The message will look just like an email with a WAV attachment to Outlook/Exchange. Replies and forwards would be just simply email at that point. If you use the Cisco VMO player, the form would be loaded then and email response then would be re-routed to Connection and Connection would proxy via SMPT to Exchange.
I have received mixed feedback from TAC/Cisco/Users so its still a bit confusing what actually happens. Assuming an LDAP integration it would make sense that UnityConnection would "know" the email address of the UC user/inbox and forward/relay to exchange. I'm wondering if the SMTP relay is used for this?
If your email client delivers an email to UC that is not a UC user email address ie firstname.lastname@example.org then UC will use the smarthost to forward the message on back into corperate email. I could do that to forward a voicemail to say my gmail account. Even if I send it to the UC server, UC knows it's not gmail.com so it looks for a smarthost configuration and forwards the email to them to take care of delivery for us.
So if you try to forward a vm message to an email address that isn't a UC address it will still end up getting delivered. UC isn't guessing the email address, you typed it into the email when you hit the forward button.
At that point though the message isn't really much of a voicemail, it's more of an email with an audio attachment.