Actually if you read my previous post, its pretty clear. But anyways.. here goes
This mainly addresses the from address showing up as the smarthost user account, also gives an explanation on how and why you get auto archives.
First off you have to understand that with a google account, anything you send is basically sent as a sent from that account.
Therefore if you are using
A@B.COM as the smarthost user, then the email will be sent from that address irrespective of the actual user (behind the firewall)
This also means the email will be in the sent folder of the aforementioned address.
So therefore if you use this with google /google apps based account, you will automagically compulsorily get a copy in the sent folder, irrespective of what you do. Which in a way is great if you want to have an archive account. For the sake of making things easier, lets actually call this the archive account henceforth.
If you want to allow the users email to go through, then login to the archive account, and then go to the settings. In the identities, you need to set up each user account as part of the archive account. This basically confirms with google that the archive account is ALLOWED to send mails on behalf of the other accounts (users).
To confirm this process the user account will receive an email, which must be accepted.
Henceforth any email from that user will automatically go through without having the from address changed.
Hope this makes life easy for you