This article explains how to change the account order in Outlook 2010 and later, and in Outlook 2007.
How do I change the order of email accounts in Outlook?
If you use Outlook to access multiple email accounts, you may want to see them in a different order than Outlook displays them. If you use the Unified Inbox in recent versions of Outlook, you can receive email sorted by email account.
Starting with Office 2010, arranging how your email accounts appear in Outlook is as simple as dragging and dropping them into the order you want them to appear with your mouse. This process is much easier if you collapse your accounts beforehand, making them easier to sort.
This is how it works:
