Microsoft Exchange Server refuses to send mail to .net domains

mails sent to domains other thann .com are not delivered:
the message is”
Your message did not reach some or all of the intended recipients.

Subject: 08077478478
Sent: 2/29/2008 9:44 AM

The following recipient(s) cannot be reached:

infocenter@nsl1.net on 2/29/2008 10:03 AM
There was a SMTP communication problem with the recipient’s email server. Please contact your system administrator
”

where in MS exchange do effect settings to allow mails to certain other domain