Out email server will only relay addresses from our email domain. I therefore need to be able to change the default "Email From" address to one of our addresses. Otherwise our email server refuses to send the message. How can I change the default from address for this purpose?

Under Tools => Manage => Email Templates, in the upper right there is a button called 'Update Email From for All Templates'.  This will update the "From" address for all outgoing emails aside from the Auto-Reply email, configured under Tools => Manage => Email Accounts, under the 'Auto Reply' tab for a configured email account.