The specified string is not in the form required for an e-mail address

I have an email form that gets all users and sends an email.
This code used to work ok for me though a little slow.  Now, it doesn't work and I get the following error:
The specified string is not in the form required for an e-mail address [FormatException: The specified string is not in the form required for an e-mail address.] System.Net.Mime.MailBnfHelper.ReadMailAddress(String data, Int32& offset, String& displayName) +1209 System.Net.Mail.MailAddress.ParseValue(String address) +240 System.Net.Mail.MailAddress..ctor(String address, String displayName, Encoding displayNameEncoding) +85 System.Net.Mail.MailAddress..ctor(String address, String displayName) +10 broadcast.Send(String Sender, String SenderDisplayName, IList Recivers) +295 broadcast.PostCommentButton_Click(Object sender, EventArgs e) +39 System.Web.UI.WebControls.LinkButton.OnClick(EventArgs e)  ...