Vista - CDO Problem

I'm trying to configure a script for a client so that he can send messages
while away from the office. The following batch file works nicely,
regardless of his location:
@echo off
set Sender=me@xxxxxx_isp.com
set Host=mail.office_isp.com
set User=me@xxxxxx_isp.com
set Password=MyPassword
set To=me@xxxxxx_isp.com
blat.exe test.txt -server %Host% -subject Test -f %Sender% -to %To% -U
%User% -PW %password%

The following script file works while he is in the office but fails when he
is on site:

schema = "http://schemas.microsoft.com/cdo/configuration/"
Set objEmail = CreateObject("CDO.Message")
With objEmail
.From = "me@xxxxxx_isp.com"
.To = "me@xxxxxx_isp.com"
.Subject = "Test Mail"
.Textbody = "The quick brown fox"
With .Configuration.Fields
.Item (schema & "sendusing") = 2
.Item (schema & "smtpserver") = "mail.office_isp.com"
.Item (schema & "smtpserverport") = 25
.Item (schema & "smtpauthenticate") = cdoBasic
.Item (schema & "sendusername") = "me@xxxxxx_isp.com"
.Item (schema & "smtpaccountname") = "me@xxxxxx_isp.com"
.Item (schema & "sendpassword") = "MyPassword"
End With
.Configuration.Fields.Update
.Send
End With

The error message is:
The server rejected one or more recipient addresses. The server response
was: 554 5.7.1 <82-220-76-236.client_isp.com[82.220.76.236]>: Client host
rejected: Access denied

Any thoughts as to why the batch file works and the script fails, even
though it uses exactly the same parameters?