Microsoft Exchange 2000

Whenever i try to send mail to a particular domain, the mail bounces back with this error message, “Your message did not reach some or all of the intended recipients.

Subject: test
Sent: 12/8/2006 8:10 AM

The following recipient(s) could not be reached:

‘info@armonkindoor.com’ on 12/8/2006 8:10 AM
You do not have permission to send to this recipient. For assistance, contact your system administrator.
… RTANCHR Contact postmaster@westnet.com if in error, but we refuse mail directly from hosts with generic reverse DNS; please use another outbound mail server. (static)>”

Can you help?

Thx