How to setup K9 Mail properly for free email services offering multiple aliases for same email box?
In K-9, when in your Inbox, hit the Menu key and chose Settings→Account→Compose→Identities. You should see a "Standard Identity" already in place. Hit Menu again, and chose to add a new identity. Think of "identity" as a synonym to your "alias".

I've not used this feature myself, but reports say K-9 also automatically choses the correct alias when replying to a mail.