Gmail's Limitations for Sending Messages

Some information from Google Apps Team, the purpose of the limitation is to prevent the spam mails I guess.

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)

Please note that there may be additional factors which slightly raise
or lower these limits, but this should be a good general guideline.

Gmail help center

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.

Bulk Senders Guidelines