Starting around 3:30PM PT 3/2/21 through approximately 9:40AM PT 3/3/21 customers may have experienced issues when attempting to send a Single Send to All Contacts through the UI. The error message received would have been "Unable to send Single Send". Engineers have determined the cause and put a fix in place. At this time customers should be able to send to All Contacts successfully. If you are still experiencing this error when attempting to send to All Contacts, please refresh your screen and single send editor to allow the code fix to load, then attempt your send again. This issue only affected New Marketing Campaign Single Sends, Automation was not affected nor was Legacy Marketing Campaigns. We apologize for any inconvenience and thank you for your patience.