I just found that mail 6.2 uses the account that is the topmost account in your inbox list. So just move the business account to the top position by drag and drop in the regular mail 6.2 window - and it works.
It is strange that this order of accounts can not be changed in the preferences. So could be considered a minor bug.
You should be able to open the "Mail" app>then click on the word "Mail" in the upper left corner of the desktop in the menu Bar, and then click on "Preferences">click on the "Composing" tab>in the line below that reads "Send New Messages From", click on the arrows at the right end of its box to open the drop down box, and there select your Business Account from the list. That should solve the problem:
Message was edited by: Radiation Mac