I've reported this as a bug (http://support.dotnetnuke.com/issue/V...) as I can not appear to find anything configuration-wise to resolve this.

Does anyone have any ideas on how to troubleshoot this of configuration settings that may allow email to start working?

I want notice to you (how I've explain at the start of this post) that this exception occour only clicking on test button in SMTP host settings. Everything else that works with SMTP work fine (for example password reminder, or add role notification).
I also want notice to you that I've this issue with a new DNN 5.6 installation, and mail account of admin user is the same of host user.
Below you can see the exception I can find in log viewer about this:

Method: System.Net.Mail.SmtpTransport.SendMail
StackTrace:
Message: System.Net.Mail.SmtpFailedRecipientException: Cassetta postale non disponibile. Risposta del server: Delivery is not allowed to this address. in System.Net.Mail.SmtpTransport.SendMail(MailAddress sender, MailAddressCollection recipients, String deliveryNotify, SmtpFailedRecipientException& exception) in System.Net.Mail.SmtpClient.Send(MailMessage message) in DotNetNuke.Services.Mail.Mail.SendMail(String MailFrom, String MailTo, String Cc, String Bcc, String ReplyTo, MailPriority Priority, String Subject, MailFormat BodyFormat, Encoding BodyEncoding, String Body, List`1 Attachments, String SMTPServer, String SMTPAuthentication, String SMTPUsername, String SMTPPassword, Boolean SMTPEnableSSL)

I've log my server mail to understand if this test email never left out DNN but I have not found anything (I'm also the administrator of this server mail and I've access to mail traffic log)

http://www.comanswer.com/question/send-smtp-email-through-godaddy