Difficulty in sending e-mail

I tried to send e-mails to 600 FISCA members from a web application. It took a long time to send out the e-mails and is blocked by Google after 500 mails. There are other obstacles in sending which are not transparent to me. Some of the messages were sent out more than once.

It seemed that the only way to overcome this problem is to pay a third party mailing service at 10 cents per mail to send them out. Do you know of any better way? Send your solution to kinlian@gmail.com. Thanks.