Hi,

I'm not a real expert in this, but it seems that the message is coming from your mail server, and it is trying to "protect" itself from spamming.

Probably the mail server is set to accept mail to send, only from inside your domain after a required lookup. But (and there could be many reason for that) the server it is not able to get the lookup of your machine, so it is denying the use of that server, thinking that you are not part of the domain.