Last October, Google announced a new administrator notification which lets you know when one of your users is abusing the SMTP relay service to send out spam or malicious emails. If you are a super administrator at your domain, then you already receive a list of the users affected, steps to identify why it’s occurring, and how to fix it. After February 6, if Google still detect abuse after 24 hours, your user being flagged for abuse will be automatically suspended from sending messages in order to prevent further domain reputation damage. You will be able to re-enable these accounts from the Admin console, but you should first determine the source of the spam.
More Information: Help Center