Setting up Google Mail to allow sending from another account
By default Gmail or a Google App's account email system will only allow you to send emails, with the email account you have logged in as, with the same email. In some cases you might have another email that you would like to use as the sender instead.
This is possible in your Google Apps/Gmail account by simply updating your settings to let Google know that you might send emails using this other email address as your sender.
Read Google's support article on how to do this here...http://support.google.com/mail/bin/answer.py?hl=en&answer=22370