Using a different "From:" email address

Hello,

I have a Hotmail account that I use for my primary email address (myaccount@hotmail.com).

I recently began doing parttime work for my friend's online store. She assigned me a shop domain email address, but I think it's not a "real" email account--just a redirect. She gave me (employee@shopname.com). It was set up to redirect to (myaccount@hotmail.com).

Everything is working correctly with the redirect. Here's the issue I'm having--she wants me to use my (employee@shopname.com) when I am sending email on behalf of her store.

Since (employee@shopname.com) isn't a "real" email address, I can't add it to my Outlook. That wouldn't be so much of a problem, except that she complains is when I initiate emails to clients AND when I reply to emails that people send to my (employee@shopname.com) email, the "From:" box lists my (myaccount@hotmail.com) address. She wants all of my emails to look like they came from (employee@shopname.com).

Is there anyway to change this setting via Outlook? How do I make it look like all my emails come from the (employee@shopname.com) email address?

Thanks, in advance, for any help.