Service Client Receives "Bad Request (Invalid Header Name)" Respon

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I have written a Web Service that works with my test harness written using
VS2005. However, a client to the service consistently receives a "Bad
Request (Invalid Header Name)" response for each request. He is using
another development environment that requires that he create the actual
request via multiple calls to a library to construct the request. I have
sniffed the traffic from my harness program and the requests look like the
following:

POST /TotalService/POSService.asmx HTTP/1.1
User-Agent: Mozilla/4.0 (compatible; MSIE 6.0; MS Web Services Client
Protocol 2.0.50727.1433)
Content-Type: text/xml; charset=utf-8
SOAPAction: "http://www.coltpos.com/TotalService/ExecuteBasic"
Host: www.coltpos.com
Content-Length: 1126
Expect: 100-continue
Connection: Keep-Alive

HTTP/1.1 100 Continue
<?xml version="1.0" encoding="utf-8"?>
<soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:xsd="http://www.w3.org/2001/XMLSchema">

[Mr. Arnold]

"Ea******@HighFlyingBirds.com"
<Ea************************@discussions.microsoft. comwrote in message
news:EF**********************************@microsof t.com...

<snipped>

>
His responses are:

HTTP/1.1 400 Bad Request
Content-Type: text/html
Date: Mon, 18 Aug 2008 11:21:45 GMT
Connection: close
Content-Length: 42

<h1>Bad Request (Invalid Header Name)</h1>

http://support.microsoft.com/kb/828726

Maybe, you need to start looking at the raw HTML/data that's being sent.