If this was working recently and nothing has changed on your WeBWorK server then there is a good chance that it has something to do with your email provider. Have you checked with your IT folks to see if they have changed any security settings on their SMTP servers? You may need to have your WeBWorK server whitelisted to send messages.
Another thought: do @kpu.ca and @student.kpu.ca email addresses use different SMTP servers? It's possible that the SMTP server that you have set in the WeBWorK config is now restricted to @kpu.ca email addresses, and is therefore rejecting messages from @student.kpu.ca email addresses.