System Format exception sending emal from a form

Hello I am new to ASP.net and am trying to include a form on my site. I have gotten the form to run on the development server . Display a reply message but when I add the code to send the message I get  System Format exception message..   string is not in the form  required for an e-mail address ..    code line is....   msg = new MailMessage(txtEmail.Text, "questions@my-site.com", txtMessage.Text, sb.ToString());  .....

questions@ my server has a Hyphen in the address could this be the problem ? Have searched the forum and other sources cannot seem to reslove this problem. Any suggestions would be helpful.

Thanks Pat F.
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