A new feature has been added to Release 10.0.21 to Email parameters to configure email throttling by email provider for both Exchange and SMTP providers. This feature limits number of outgoing emails per minute.
The default is set to 30 email per minute which makes sense to Exchange and Office 365 email providers as this is the maximum number of emails per minute Exchange will allow you to send emails out.
For SMTP provider, depending on the service the maximum number of emails sent out per minute can vary, but for sure it is more than 30. So remember to configure it correctly if you have a batch process that send out emails out of Dynamics 365.
AZ on D365 