I took to using Fiddler to sniff the HTTPS traffic and found the following:
URL = https://www.cfhdocmail.com/Test_SimpleAPI/DocMail.SimpleAPI.asmx
Method = SendLetterToSingleAddress_FromURL_TextResponse
POST http://www.cfhdocmail.com/Test_SimpleAPI/DocMail.SimpleAPI.asmx HTTP/1.1
Changing the URL constant affects the target POST address, so that resulty in a 404 at tempuri.org.
Using Fiddler's request builder I created the following which results in a successful exection:
POST https://www.cfhdocmail.com/Test_SimpleAPI/DocMail.SimpleAPI.asmx HTTP/1.1
Basically, it works if the the namespace is taken from the definition rather than the URL. Admitedly this webservice has left the default namespace instead of defining it's own, but it's potentially valid to have a target URL that differs from the URI of the namespace.
Not sure if there's a quick workaround, but hopefully the info aboeve helps.
For best results, use the Firefox browser..