This answer not coming from Microsoft, but it sounds reasonable.
When you have multiple accounts configured in Outlook, the account that's used to send mail varies based on several things:
• When you compose a new email, Outlook uses the email account that you've configured as your default.
• When you reply or forward an email, Outlook uses the email account that the message was originally received on.
• In either case, you can explicitly specify the account to use via the Accounts drop down item that appears next to the message's send button.
The original post that I read this off of came from Leo Notenmoon (http://ask-leo.com/why_is_outlook_sending_email_using_the_wrong_account.html)
Class: Information Worker: Outlook 2007 Power User*
Date: November 12, 2008
Location: Indiana State University
*The Information Worker series is available through LanTech Training in Indianapolis. Please visit their website for more information