I have a similar setup for receiving my Exchange mail on my iPhone, only I use a GMail account.
The problems I had with using the Yahoo accounts were 1) all of the marketing messages being added to the bottom of each and every email, and 2) mail being sent from me@yahoo.com instead of me@companyname.com.
To get around this, I have the following set up:
1) Had my IT department setup my Exchange mail delivery so mail is delivered to both my Exchange box and my Gmail account. (First, set up the new address in AD. Then, in your Exchange account's Delivery Options, choose to forward mail to this address. Also check the box to Deliver Mail To Both Forwarding Address And Mailbox)
2) Configured my Gmail account to be able to send emails using my work email address. Also set Gmail to use this address as my Reply-To address (Located in Settings-->Accounts within Gmail)
3) Confiured Gmail access on my iPhone. I believe I ended up setting this up manually for some reason. I vaugely recall it having something to do with configuring Outbound SSL correctly.
So now I can receive and send emails from my iPhone and have it appear as if they're coming from Outlook, without any marketing messages.
My next challenge was to set up syncing of contacts/calendars between the iPhone, iCal and Outlook. For this, I use Plaxo (
http://www.plaxo.com). It installs a service on both my PowerMac and my laptop. Works like a champ.