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You can use the Inbox to send a message to all users in a course, all users in a specific role, or all users in a group. If your recipient list contains more than 100 users, your message will automatically be sent as individual messages to each user. As the sender, you will also be included in the total recipient count. Detailed instructions can be found in the following Canvas Guide, How do I send a message to all course users in the Inbox as an instructor?