So do you have 2 domains in Google Apps and you don’t want to be logging on and off every time?
You can allow one of them to send email on behalf of the other (without showing the “on behalf of” message)
Open the mail account that you want to have as primary, go to Settings > Accounts: and press “Add another email address you own”
A new window will pop up:
Write down the name you want to appear when the receiver receives your email, write the email, untick the “Treat as an alias”, and press “Next step”; a new window will show:
Here you select “Send through example.com SMTP servers”, and fill the fields:
SMTP Server: smtp.gmail.com // Select port 587
Username: (the email in question)
Password: (the password)
Select “Secured connection using TLS” and press “Add Account“. If everything was OK, then the window should close automatically, otherwise will stay and you should revise the details.
With this done you will be able to send emails even for domains using Google Apps.