A 601 error is a generic error, and just means that the attempt to send the message failed. To determine the detailed failure information you'll need to go into Manage>View Logs and look up the Delivery logs that relate to the to address that failed delivery. This should return the SMTP session that failed and allow you to see the exact error that caused the 601 error.
Kyle Kerst
Lead Internal Network/System Administrator
SmarterTools Inc.
smartertools.com