SMTP Connection

Hi!

There are two Exchange 2K server in one org, Ex1 and Ex2, in the same routing group. There was no problems, but suddenly, Ex2 is not able to receive from anybody, but able to send email to outside and Ex1. When I sent email to an account located on Ex2, I got this message.
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Your message did not reach some or all of the intended recipients.

Subject: test
Sent: 6/20/2003 4:17 PM

The following recipient(s) could not be reached:

testuser on6/20/2003 4:18 PM
The recipient could not be processed because it would violate the security policy in force

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Any suggestions or solutions will be very appreciated.
Thanks.