Error Using SMTP Server From the CSK

I have been unable to use the outgoing email options of the CSK and when attempting to do sothe following exception is generated:

Inner Exception:
The server rejected one or more recipient addresses. The server response was: 553 sorry, that domain isn't in my list of allowed rcpthosts (#5.5.3 - chkuser)

I changed the SMTP server name from mail.learningce.com to localhost and this seemed to fix the problem. I was just wondering why this is the case. Isn't better that I use mail.learningce.com?

Jim