Once the domain has been successfully added and verified, click Skip Google MX setup for your secondary domain or domain alias (listed on the Domain settings page).
To verify that you are routing your mail to another server, click I use another mail server.
The status for your domain will then change to Active.
Note: This setting enables you to activate the domain even if you are routing its email through a mail server that's not hosted by Google. This might be your own on-premise server, or a third-party service that provides email archiving or spam protection.