Actually I had just spotted a warning about exactly that in dashboard, and so clicked the handy provided link to appropriate setting and ticked the box.
I had not realised that force https was mandatory for receiving incoming email
Just possible that lack of enforced https has also been causing issues with using facebook login - I have recently been notified by Facebook that my site was in breach of their terms of service and has been suspended. There were no action items in my Facebook apps developer control panel, so I appealed, and the response was that they could not verify the site due to an unspecified error generated by my forum url,