This is very helpful, and confirming my understanding of the bug.
The mistake is using the target URL as the basis for calculating the soapAction, instead of using the soapAction specified in the WSDL file.
Thus, in your case, the target URL ("https://www.cfhdocmail.com/Test_SimpleAPI/DocMail.SimpleAPI.asmx") + method name ("SendLetterToSingleAddress_FromURL_TextResponse") give us:
instead of the correct soapAction as defined in http://www.cfhdocmail.com/Live_SimpleAPI/DocMail.SimpleAPI.asmx?WSDL:
The solution should be a correction of the WSDL import wizard. I don't see a workaround.
I hope to address the bug soon.
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