Help me understand Email Messaging pricing
The pricing plans are available here:

You will see Email under each plan. This is how many times you can send out email messages in a month. One use event occurs each time there's an outgoing email message. For example, in a scenario where you email to the learners the typed responses they submitted in the course, and 5 learners take your course one time each: there will be 5 outgoing email messages, and you will use 5 credits.

You can track your current usage and your remaining credits on this page:

There is a default email notification that goes out when you exhaust 80% of the credits for the month, so this will give you enough time to make arrangements if needed. You can choose to upgrade to the next plan if you’d like to keep using email messaging in your courses or do nothing. If you run out of credits, you will not be able to send out messages for the remainder of the billing period, and the credits will reset with the next billing cycle. You can verify that the notification is turned on (or turn it off if needed) on the Account page.