In the webmail system, you can set up multiple From: email addresses for outgoing messages, by creating or editing Identities under Preferences > Mail > Personal Information.
When you add a new identity with a new email address, a confirmation message will be sent to that email address to confirm that you own that address and can receive mail there. Once you confirm the new email address it will be added to your list of identities and you will be able to select it as a From address when sending mail through the webmail system.
You can add any number of identities, and delete them as you like. Valid email addresses for identities include your subdomain alias addresses, your domain addresses (if your account is configured to use a custom domain) or even addresses you have at other email services.
Most email programs can also be set up so that you can select from among multiple identities for the "From:" header. Setting this up with Thunderbird is described in another question in the "Sending Mail" section of the FAQ. We're working on publishing more information about how to do this with other email programs.