Thanks Cesar.
The only error I can see is in the Error Log for the /MailgunSample/Mailgun.aspx screen when I click the "Send a test email" button. The stack trace is:
Error generating the email!
at ssMailgun.Actions.ActionSendMimeEmail(HeContext heContext, String inParamApiKey, String inParamDomain, String inParamFrom, String inParamTo, String inParamCc, String inParamBcc, Boolean inParamLogContent, String inParamEmailName, String inParameSpaceName, Int32& outParamSent_EmailId, String& outParamMessageId, String& outParamResult)
at ssMailgunSample.RsseSpaceMailgun.MssSendMimeEmail(HeContext heContext, String inParamApiKey, String inParamDomain, String inParamFrom, String inParamTo, String inParamCc, String inParamBcc, Boolean inParamLogContent, String inParamEmailName, String inParameSpaceName, Int32& outParamSent_EmailId, String& outParamMessageId, String& outParamResult)
at ssMailgunSample.Flows.FlowMainFlow.ScrnMailgun.CommandSendEmail(HeContext heContext)
The Email Log does not have an entry for the email, so it doesn't seem to get as far as rendering the email message.