Did you get an answer to this? It sounds like you want to click the little button in Accounts so that when you reply to an email sent to your alias, it automatically comes from the alias email. Alternately, you can leave the button selected that sends every mail from your main email, but in the individual reply, you can click "change" next to your email address and you get a drop-down menu where you can select what email you will be replying from. That leaves room for forgetting, though, so I prefer to have emails reply from the address they were sent to.