"outdated version of mail-receiver" in Dashboard

I’m not sure what kind of test you are looking for? You can look in your admin Dashboards and see if it says “outdated verson of mail-receiver…” this will only trigger if it detects it using the old api auth method. If your sites aren’t using the mail receiver or they haven’t received a single email this message will not display. So I guess as a test you could reply to a private pm via email and see if it shows up?