We are currently aware of a global issue affecting emails sent to Microsoft-hosted domains (such as hotmail.com, outlook.com, live.com, etc.). Emails affected will show as "Processed" on the "Reports > Activity" page, and delivery attempts encounter "451 4.7.650……….." responses. This is a Microsoft-wide issue affecting multiple SMTP providers/ESPs, and we are monitoring it closely as they actively work on a fix. Thanks for your patience.