A quick note that our servers are rejecting mail from calendarnotification(AT)email.microsoft.com as they currently fail Microsoft's own SPF policy for email.microsoft.com.
These mails could be bogus or could be genuine. The SPF error is what makes our decision to reject an easy one.
Currently the SPF record reads:
and mails are being sent from: 65.54.190.226 (bay0-omc4-s24.bay0.hotmail.com)."v=spf1 mx:email.microsoft.com ip4:207.46.222.193/26 include:_spf-ssg-a.microsoft.com -all"