The add-on is restricted by the email limitations of Gmail accounts, described in Quotas for Google Services. For example, consumer accounts are limited to 100 email recipients per day, and maximum 50 per email message. This quota applies to all add-ons and Apps Scripts, so the number of messages you can send from the Email Range add-on may be decreased due to your other activities.
If you exceed the limit applicable to your account, you will receive an error message such as: Service invoked too many times for one day: email, and your Gmail account may be disabled for a day.
After you've sent a message from the add-on, it will check and display your remaining quota.