It appears that the cap is 100 users per email. Is this correct?
Yes, because the Users list paginates at 100 you can only send 100 emails at a time. You can send emails page-per-page if you have more than 100 users.
Or, for a more robust solution, I'd recommend integrating your site with an email marketing tool like MailChimp or ConvertKit.
Happy to help if you have any further questions at all.
You can check out this article on how to email your students from the admin dashboard: http://help.thinkific.com/support/solutions/articles/216736-mass-user-email
Currently we don't integrate with mailpoet but we do offer integration with Mailchimp, Convertkit and Aweber.