Force Outlook 2007 to send via default e-mail account

We have recently changed our e-mail addresses to our new domain - so me@olddomain.com has been replaced with me@newdomain.com.  We sent out notification of the change, but many of our customers still use the me@olddomain addresses.  

Everyone has both accounts, they are both POP.  We are using every version of Outlook from 2003 - 2007, but we can upgrade everyone to 2007 if necessary to solve this problem.

Outlook automatically uses the olddomain.com accounts to reply to these e-mails.  I understand that we can manually choose the correct box under "accounts," but this is too hard to remember for my staff.  I have found a variety of suggested solutions to this problem, none of which have worked.  I even paid for this site to view a solution that I had already found elsewhere and didn't work - to change the default reply address and SMTP server login on the old account to the same as the new default account.  

That did not work for us.  It may be because both our domains are currently hosted with yahoo, so the SMTP server is the same.  For some reason changing the logon information for sending mail on the old account to me@newdomain.com does not work.  The reply-to address change works, but mail sent still appears to be from me@olddomain.com despite supposedly logging on the server with the new account information.

Is there any way to solve this?  It is taking up way too much time to figure this out.