If you reply to an existing e-mail then Outlook will use the account that
received the e-mail. Otherwise it should use the default account.
You may want to have a look at SendAccount 4Outlook.
http://www.addins4outlook.com/sendaccount/
A common problem with using multiple accounts is accidentally sending out an
e-mail using the wrong account e.g. sending a business-mail using your
personal e-mail account.
Send Account lets you set a preferred account for each person you write to.
It warns if you if you try and send an e-mail to someone using the wrong
account.
Regards
Sanjay Singh