Sometimes you want to change the "From" address when sending email, but the address is not available. You can add an address that you own using the Google Web App (GWA). Log into the GWA and click on the gear icon in the upper right. Select Settings and then the Accounts tab. In the "Send mail as" section, first look to see if the address is already there. If it is, simply click on Make default to the right of it and it will be your default "From" address.
If the additional address is not there, click on Add another email address. Enter your name and the address you want to send from. Click Next Step. If needed, enter the SMTP server (for example, smtp.domain.com) and the username and password on that account. Click Add Account. To confirm that you own that address, log into that account, open the confirmation message, and click the link.
If you are using Microsoft Outlook with GASMO/GSSMO, the new address will be synced to Outlook by restarting Outlook and/or by waiting for 10-15 minutes.