Locked out because of a quota issue in email.
Getting locked out of an account can be a frustrating experience. Without looking into the account, I can't comment on the root cause, however I encourage anyone with quota issues to contact support to learn why this particular issue occurred. Furthermore, please note that these lockouts are designed to protect a user's accounts.
Limits and quotas protect the Google infrastructure from an automated process that uses the Provisioning API in an inappropriate way. Excessive requests from an API might result from a harmless typo, or may result from an inefficiently designed system that makes needless API calls. Regardless of the cause, blocking traffic from a specific source once it reaches a certain level is necessary for the overall health of the Google Apps system. It ensures that one developer's actions can not negatively impact the larger community.
Although this speaks directly to the Provisioning API, the spirit applies to other services as well. We have quotas so that everyone has a good experience with Google Apps. As you might expect, each application has different types of quotas in place. Here are some common quotas (all are listed in our help center).
Email sending limits: http://www.google.com/support/a/bin/answer.py?answer=166852
Google Calendars: http://www.google.com/support/calendar/bin/answer.py?answer=37082
Google Docs: https://docs.google.com/support/bin/answer.py?answer=37603