Web Services Addressing SOAP Binding spec section 4.2 :
"Use of the SOAPAction HTTP request header field is required when using the SOAP 1.1 HTTP binding. The field-value of the SOAPAction HTTP request header MUST either be the value of the [action] property enclosed in quotation marks, or the empty value "". The latter case supports the ability to obscure the [action] property through SOAP-level security mechanisms, without requiring otherwise unnecessary transport-level security. Any other value for SOAPAction results in an Invalid Message Addressing Property fault."