HTTP 400 Bad Request caused by To and Action in Soap Envelope head

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I get HTTP status 400 - Bad Request when I send the following Soap envelope
to a local WCF web service. If I either remove both the To and Action
elements or the whole Header element, I get a good response. Any clues why
the WCF service does this?

<s:Envelope xmlns:s="http://schemas.xmlsoap.org/soap/envelope/">
<s:Header>
<To s:mustUnderstand="1"
xmlns="http://schemas.microsoft.com/ws/2005/05/addressing/none">http://localhost/DevWcf/WcfSimpleTest/WcfSimpleTest.svc</To>
<Action s:mustUnderstand="1"
xmlns="http://schemas.microsoft.com/ws/2005/05/addressing/none">http://tempuri.org/ISimpleTest/SimpleOperation</Action>
</s:Header>
<s:Body>
<SimpleOperation xmlns="http://tempuri.org/">
<SimpleValue>test string</SimpleValue>
</SimpleOperation>
</s:Body>
</s:Envelope>