From: DeskNow Knowledge Base
http://www.desknow.com/kb/

550 Dynamic IPs/Open relays blocked
http://www.desknow.com/kb/idx/11/032/article/

This error can appear when the mail server of a recipient blocks mail coming from a mail server (your DeskNow server) that has a dynamic IP address.

The dynamic IP address is assigned to your computer by your ISP, when you login with your modem.

This is an anti-spam measure that some system administrators decide to take.

Solutions

1) ask your ISP to give you a static IP address, or change ISP

or


2) use SMTP forwarding to send mail through your ISP