I suspect there is a bug in the WSDL import.
Please try to replace the value of the URL constant in SendLetterToSingleAddress_FromURL.soap from "http://www.cfhdocmail.com/Live_SimpleAPI/DocMail.SimpleAPI.asmx" to "http://tempuri.org/".
If it doesn't work, please attach a sample project where you have HTTP Post working and SOAP call that doesn't work (a .tersus file). To export your project, stop the application, right click the project, select Export..., select Tersus Project File, and press Next and then Finish.
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