@pissbabybrown - It sounds like you've done what is necessary to set up an email forwarding account. If you're still having trouble, please consider sharing the actual domain you're using. This might help others offer suggestions on how to move forward.
@YC-CHEN - To clarify, our email forwarding service can only be used to forward an email to another active email address. If you want to have the forward send from @123.com to an @gmail.com email address, that can be done using our forwarding service. However, if you want to use @123.com to send email using Gmail, this would not be an option for you. In that case, you will need to set up email service with Gmail.