This question was just asked in the Exchange Admin group
(microsoft.public.exchange.admin) about an hour ago.
The short answer is that by default you can't send mail as a proxy address
when using Outlook in MAPI mode. It's something that people have wanted for
ages, but has never been implemented.
There is a product callled ChooseFrom by Ivasoft that will permit this.
http://www.ivasoft.biz/choosefrom.shtml (link courtesy of Andy David in the
Exchange group)
The alternative would be to make the proxy address its own mail-enabled
account and give the user Send As permissions on that account, then the
sender would manually type in that address in the From: field when sending
mail as that address. Not a great option, the Ivasoft is probably the way
to go. This is how I did it for one of my users who wanted to both receive
and send as some email accounts we'd set up for various internships.
There's also the possibility of not using Outlook in MAPI mode, and using
SMTP to send mail instead. This allows you to enter whatever email address
you want when sending mail, as it's not integrating with Exchange when doing
so. :/
Joe