A bug introduced this afternoon in our outbound mail filters would, under certain circumstances, add a non-compliant header to messages sent through mail.sonic.net.  While most servers we’re happy to accept these messages some systems refused to accept the messages.  A fix for the bug has already been released into production.  –Kelsey
On Saturday, July 26, Earthlink is bouncing all email sent through c.mail.sonic.net. Is this related to the earlier bug or a separate problem?