microsoft.public.exchange.setup

Hi,
I have setup a new domain (xyz.com) in my Exchange 2007 by creating
the accepted domain and email address policy for the new domain. I
changed the M record accordingly.

When I look at users' properties, under Email Address tab, they have
both abc.com (existing email) and the new xyz.com email addresses.
abc.com is set to be the default email address.

Users get both emails in their Outlook. But I am not sure how to set
it up so that 1) they can select which address they want the send
their emails from, and 2) how can they reply to an email and have the
"Sent" field specify the email address that it was received by.
I cannot setup a second email account since only one Exchange email
address can be set up.