**Message you sent blocked by our bulk email filter**

My endusers occationaly get this message when they come into work in the morning and I’m not sure why this happens.
From: System Administrator
Sent: Wednesday, August 08, 2007 7:18 AM
To: Scott
Subject: Undeliverable: **Message you sent blocked by our bulk email filter**

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

Subject: RE:
Sent: 8/8/2007 7:06 AM

The following recipient(s) cannot be reached:

scott.rudderham@canpotex.com on 8/8/2007 7:18 AM
You do not have permission to send to this recipient. For assistance, contact your system administrator.
< spamfirewall.ext.canpotex.com #5.7.1 smtp; 550 5.7.1 Message content rejected, UBE, id=29856-02-25>

Can someone help me out here?
Thx – John Goroff