Handling multiple accounts in Outlook

I have two accounts setup in Office 365, each with different domains. Neither are "Shared" mailboxes. I have to manually change the From address for both new emails and replies. Prior to the Office 365 migration (from Exchange 2003) Outlook 2010 would automatically change the From address based on which account folder was selected at the time. Is there a way to revert to this behavior?

Note that the "Always use default account.." option is not enabled in Outlook.

Thank you.