Well I may have been unclear about the inbound versus outbound part. You can do as much inbound or outbound email that you want.
The Recipient Policy which I mentioned earlier, has several jobs:
a) defines what email domains your exchange organization receives email for
b) defines any number of automatically generated email addresses and/or alias' for a given mailbox
c) defines the "primary email address" which appears as the sender email address of outbound email
So the comment relative to inbound/outbound is pertinent in this way: you can have any number of email domains and/or alias' for which inbound email will wind up in a given mailbox, however, of course an outbound email will only advertise one email address as the sender, and in exchange this is called the Primary email address.
In your situation, I'm assuming that you have your own comany domain that you use as your email domain, call it @yourcompany.com. Per your partner company request however, presumably they would want replies to inbound partner requests have a sender email address of @partner.theircompany.com, if that makes sense.
So in Exchange, although the same user mailbox can RECEIVE email addressed either way, that mailbox user will send OUTBOUND email as johndoe@yourcompany.com OR johndoe@partner.theircompany.com, whichever is set as the Primary email address, not both. There is no way in Exchange to toggle, say a "send as" setting as you may have seen in some email clients.
To manage Exchange in your environment so that your company does not lose its own identity to the partner email domain, you would have to
a) use separate email boxes for partner email use, and manage the multiple mailboxes either with OWA or separate Outlook profiles, etc.
b) assign a team of users that assume the partner identity and have them handle all partner communication.
Hope this clears things up.
