Google Apps Gmail has sending limit of 500 messages per day for free gmail accounts.
Paid Google Apps accounts have limit of 2000 messages per day.
Here is the explanation of Google Apps Gmail sending limits:
https://support.google.com/a/answer/166852 (see Daily sending limit section)
Google resets this limit every 24hours. So you can still queue messages more than the allowed limit but cloudHQ can only send them once the limit has been reset by Google.