None of the address fields (To:, cc:, or bcc:) will hold more than 4,100 characters. If you go over that limit, your message won’t reach all your intended recipients—and you won’t be notified of the delivery failures. You can safely send messages to up to 100 students at one time, but unfortunately you cannot set the Email tool to show 100 users per page. If you have problems sending to 100 students, you may want to only send to 50 students at one time. A good workaround is to use the Classlist tool instead of the Email tool. Just click the Classlist link on your course homepage, change the paging to 100 users per page, and then click Email everyone on this page. Repeat for each page of recipients.