Send emails from Domain alias in google apps

From the Gmail help on my Google Apps account:

1. Sign in to your Google Apps email.
2. Click the gear icon in the upper-right and select Gmail settings along the top of any page, and select the Accounts tab.
3. Under Send mail as, click Add another email address.
4. In the Email address field, enter your name and alternate email address.
5. Click Next Step >> and then click Send Verification. Gmail will send a verification message to your other email address to confirm that you own it.
6. Locate the message from Gmail. Open it and and either click the link contained in the message or enter the confirmation code in the Accounts section of your Google Apps email settings.

So, if your domain name is mydomainname.com and your alias was alias.mydomainname.com then [email protected] would have to add a send-to address for [email protected].

Go to

Settings > Accounts and Import > Send mail as > Click "Send mail as another address"

after that it should be self explanatory.