Control over sender email address in default mail app?

I have configured the default mail app in Windows 8 to access a gmail account (via IMAP). My Gmail account pulls in personal and business email from two other email addresses (one with BT Internet, the other with a smaller hosting company). When I compose a new email or reply to an email using the Windows mail app I want to be able to change the "sender" address, depending on whether the email is personal or business. I have set up 2 different email aliases in my Live account, and set my personal email address (john@something.com) as the primary alias, but whenever I send emails from Windows 8 mail the reply address is simply set to "john@" (with no email domain), and I can't find any option to change it.

How can I control the sender address in the Mail app?