If you have have mutliple accounts setup in Outlook you will know that one is usually set to the default. That means you create a new email using Ctrl+N or the New E-Mail button you would expect to see the sending from account be your default account. Well, that used to be the case with Outlook 2007, but that has changed in Outlook 2010 and 2013.
Apparently Microsoft decided that even though you specified a default account, you didn’t really mean it. So, if you were in a folder from the non-default account and send new e-mail, it is now sent using that folders email account.
To prevent yourself from sending personal emails from your business account there is a registry setting that can be made to fix this problem.
or for Outlook 2013