Missing 'Send Notification Email' task checkbox

A sales rep reported today that he uses the "Send Notification Email' checkbox all the time when editing tasks. However, he said for some reason today he can no longer see the checkbox. I checked the task page layout properties and I don't even see the option to disable it - I only see the Page Layout Name.

Does anyone have any ideas why the checkbox has disappeared, and why I don't see the option to enable it?

Thanks!