The possible causes for receiving this error message from Google are:
- You sent a large number of undeliverable messages (messages that bounced back).
- You have linked a POP or IMAP client (e.g.: Microsoft Outlook or Apple Mail) to your Gmail account. You may then only send a message to 100 people at a time.
- You have a reply-to address from a different domain.
- You are sending many emails with another tool (keep in mind that the limit is 500 emails/day with a normal Gmail account and 2000 emails/day with G Suite)
- You are leaving multiple instances of Gmail open
More information here: https://support.google.com/mail/answer/22839
If you use G Suite please look over here: https://support.google.com/a/answer/166852?hl=en