Ever since switching to smartermail my users are constantly getting: 

 

Remote host said: 601 Attempted to send the message to the following ip's ERRORS ... With the IP addresses of the destination mail servers listed. Sometimes if you send the email again right away it goes through - sometimes it bounces a handful of times. If you sent it from Yahoo, Gmail, etc. it ALWAYS goes through right away. 

 

Any idea what can be causing this? It's becoming a an issue. 

 

I ran our domain and IP on MX toolbox and we are not blacklisted and the only "warnings" that come up are "Reverse DNS does not match SMTP Banner" and "Warning - Does not support TLS." - The SMTP banner error, could that be causing it? If so how do I fix. 

Thanks!