The different reasons to get the error message "You have reached a limit for sending mail. Your message was not sent." from Google are:
You sent a large number of undeliverable messages (hard bounces).
You have linked a POP or IMAP client (e.g.: Microsoft Outlook or Apple Mail) to your Gmail/GSuite account. You may then only send a message to 100 people at a time.
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 GSuite account).
You are leaving multiple instances of your Gmail/GSuite open on different devices.
More information can be found here: https://support.google.com/mail/answer/22839
If you use GSuite please look over here: https://support.google.com/a/answer/166852?hl=en