Thanks you guys for your thoughts and suggestions- I will say that I am very new to supporting this platform as the previous person responsible is no longer with us and we lost most of the knowledge as a result. So, thank you for bearing with me here!
When our professors have set up courses, they have never entered the email addresses, just they're university usernames, and in the past this seems to have been sufficient (and for most of the students in the class, still seems to be). I'm a bit mystified by how that is myself to be honest! I'm digging in with the professor to see if these might be newer users and if perhaps something has changed that is breaking whatever has been connecting the user accounts in webwork to their ability to email.
Barring being able to pin that down, I will check with our server admins to see if they can check the suggested items.