I typed 'mail.google.com' into the URL field, I saw it redirect to the 'accounts.google.com' URL and a block page was immediately sent from the proxy
That didn't work either - according to the trace, the referer field is still blank. Not sure what else to try!
I can fall back to the custom Gmail categories, it just would have been nice to not rely on a static list in case gmail adds a few more URLs in future!
Any additional suggestions are welcome though!