"email diff" service with private repository gives bogus mail

If you setup "email diff" with a private/paid for repository then you get an email with a mime type of text/plain, the body of the email containing a huge amount of HTML and buried somewhere in the middle of that the text "Error You do not have access to this repository".

I'm an admin for the repository so obviously have access. Presumably the cause is how you have the email diff service wired up.