To keep systems healthy and accounts safe, Google limits the number of email messages users can send per day and the number of recipients per message.
If your users exceed an email sending limit, they’ll see an error message, such as:
You have reached a limit for sending emails.
You reached a Gmail sending limit.
You exceeded the maximum recipients.
After reaching one of these limits, a user can't send new messages for up to 24 hours. However, they can still access their G Suite account and receive incoming email. The user can also access their other Google services. After this suspension period, sending limits are automatically reset and the user can resume sending mail.
Limit for recipients per message sent via SMTP and the Gmail API (addresses in the To, Cc, and Bcc fields of a single email. Includes email sent using smtp.gmail.com) - 100.