By default Windows Server 2003 Small Business Server runs a program that does a few things upon a users login. One of those things is resetting the default account in Outlook to the Exchange account, if it has one configured. This means that if you have Outlook configured with an Exchange account, regardless of how many  POP/SMTP or IMAP accounts are configured, upon logging in, the default account is always reset to the Exchange account. To disable this behavior add this Registry Value: