When a person is sending from a me.com account to one of our customers in Smartermail they are getting the following the errors returned to them.  Does anyone know what is causing these errors? We are using Smartermail 16

 

Error reading SMTP packet; response to RCPT TO command expected
 

Original-recipient: rfc822; xxxxx@xxxx.xxx
Final-recipient: rfc822; xxxxx@xxxx.xxx
Action: delayed
Status: 4.4.7 (unable to deliver this message after 2 days)