Thank you but I had seen these already and a number of other references on this topic, but none were helpful. The original post was intentionally as concise as I could make it but if you are hungry for more:
As your 3rd reference suggested I abandoned using the Apple iCloud mail server from SendMail and tried gmail instead, but with no improvement, using something like this:
SendMail[
"To" -> "someone@mac.com",
"Subject" -> "Example Message",
"Body" -> "My text",
"From" -> "someone@gmail.com",
"Server" -> "smtp.gmail.com",
"UserName" -> "someone@gmail.com",
"Password" -> Automatic,
"PortNumber" -> 587,
"ServerAuthentication" -> Automatic,
"EncryptionProtocol" -> "StartTLS"]
Mathematica returns $Failed and the message: "SendMail::msgerr: Mathematica received an unexpected response during message transport."
I don't see anything in the system logs that indicate trouble. An undocumented "verbose" option accepted by SendMail would be nice about now. Needless to say Mathematica is able to pass the internet connectivity test to the Wolfram servers, and this computer is fully functional in regard to mail by standard means.