Has anyone had any success in accessing the .Headers collection
for System.Web.Mail.MailMessage objects?

As it's an IDictionary interface, and MailMessage.Headers.IsReadOnly is 
false, I would have assumed that MailMessage.Headers.Add(key,value) would 
work. Checking the Headers collection after the .Add command does bring 
back the correct values for various keys, however the exception is being 
thrown on the SmtpMail.Send(MailMessage) command.

Thanks,

Neil