Could not deliver the output for Delivery channel:null

Hi,

I am using the XML Bursting program to send emails but it failed with null delivery channel error.  I checked the smtp server and I could send out a test email without any issues.  Below is extracted from the bursting control:

<?xml version="1.0" encoding="UTF-8" ?>
<xapi:requestset xmlns:xapi="http://xmlns.oracle.com/oxp/xapi" >

  <xapi:request select="/WEL_POXRVODS/LIST_G_VENDOR_NAME/G_VENDOR_NAME" >
    <xapi:delivery>
      <xapi:email id="123" server="<smtp server>" port="25" from="${BURST_DEL_FROM}" >
        <xapi:message id="123" to="${BURST_DEL_TO}" subject="${BURST_DEL_SUBJECT}" attachment="true" >${BURST_DEL_BODY}
</xapi:message>
      </xapi:email>