An email from Google Apps Team indicates, that Gmail has a limitations for sending messages. Here is a list of some of the sending limits in place on your account:

• 500 messages per day (i.e., you can hit 'Send' a maximum of 500
  times)
• 500 unique recipients
• 2000 total emails (for example, you could send one message to a
  group of 500 people four times)

In an effort to fight spam and prevent abuse, Google will temporarily disable your account if you send a message to more than 500 recipients or if you send a large number of undeliverable messages. If you use a POP or IMAP client (Microsoft Outlook or Apple Mail, e.g.), you may only send a message to 100 people at a time. Your account should be re-enabled within 24 hours.

If you communicate with the same group of people on a regular basis, you might be interested in Google Groups, explains Google help center.

“500 messages are nothing in this day and age, especially if you are in the information business as I am” and wonders why Google doesn't list all these restrictions.

I feel terrible for those 1.5 million Australian school kids who will now have to use Gmail instead of Exchange/Outlook. Om Malik.

Source:→ Google System