This has been happening to me. It seems to be effecting hotmail.com as the client who is not receiving invoices is on hotmail. I did a test invoice to myself on hotmail account and nothing at all comes through or in to junk. I send another test invoice to my wife's gmail email and that came straight though. I have just been on online chat and just sent the standard workaround about getting an IT expert!!

Hotmail defo seems to be an issue at the moment.

Cheers

James