Email rejected by Hotmail/MSN/Live/Outlook

We've had many reports of email unexpectedly being bounced by MSN email services, for example addresses @hotmail.co.uk, @live.co.uk and @outlook.com.  Microsoft are "looking into this issue along with the Escalations Team", but they all seem to be on holiday.  As a result we had to work around their block, but your GreenNet email to Hotmail users may still appear in the recipient's "Junk folder."