Can’t send email to another company

For some reason a few weeks ago all of a sudden, we can’t send email to a particular company. Every time we send to any user on their domain, we get the following error. I can telnet to their exchange server and manually create an email so I know that works. Any ideas?

We are running Exchange 2003 on the MS Small Business Server 2003

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Your message did not reach some or all of the intended recipients.

Subject: test from the server
Sent: 2/8/2006 1:30 PM

The following recipient(s) could not be reached:

cmorse@chbenevolent.org on 2/8/2006 1:30 PM
You do not have permission to send to this recipient. For assistance, contact your system administrator.