I'm using Google Apps Standard.
I created an alias (firstname.lastname@example.org) for my account (email@example.com). When people email me at firstname.lastname@example.org, it correctly arrives at my email@example.com email address.
How can I send mail using this alias address? That is, the firstname.lastname@example.org address should appear as the sender to whomever I email, and my real address (email@example.com) should never be revealed to them.