Gmail has certain rules in place as far sending mails is concerned. Let me point them one by one.
1. If you are using Gmail on website, you can send mails to a maximum of 500 recipients per day. Please note that the limit is on the number of recipients, not messages. That should mean if you send 50 mails to ten people, your quota is over for the day.
2. If you are using a POP/IMAP client for sending mails, your maximum limit would be 100 recipients.
3. If you send mails through Google Script, your limit will again be 100 recipients per day.
4. If you are using SMTP services, your daily limit would be 100 to 150 recipients per day if you are using a remote server.
How to avoid i?
1. The best way to avoid your account being suspended on account of more emails being sent from it, is to keep a track of how many mails you have sent.
2. Send a cc or bcc only if it is absolutely needed.
3. Avoid replying to all recipients if it not needed.
4. Double check the email address before hitting the Send button. It can save you from unneeded mails that bounce and force you send the mail again.
5. Use multiple accounts to send emails. You may create different accounts for the individual sections of your business. That is exactly why most of the organisations have multiple email accounts for each of their functions.
6. There are email marketing services like flashissue. They have their own servers through which they deliver your emails without having to go through Google's policy standards.
The Google help forums do not seem to be providing a clear idea on the exact limit and how to avoid crossing it. It would be wiser to stay within limits - or well within - so that there are no issues like your account getting banned happen. Please note that if your account is suspended for crossing sending limits, you will need to wait for 24 hours till the suspension is lifted.
All other email service providers like Yahoo or Hotmail have the similar guidelines in place. If you are using one of them, you can consider visiting their support channels to get a clear view on the limitations.
Live....and Let Live!