IMail.Forward() method creates and returns new ForwardBuilder configured to forward this email.
You can use it to overwrite default subject and body templates, and in the end to create a MailBuilder instance.
MailBuilder is used to create the final email. You can use it to modify the email in any way you want (same as you would create a new mail message).
Your code should look like this.
ForwardBuilder forwardBuilder = email.Forward();
// You can set custom templates on forwardBuilder:
//forwardBuilder.SubjectReplyTemplate = "Fwd: [Original.Subject]";
MailBuilder builder = forwardBuilder.Forward(new MailBox("valid emailname", "name"));
// You can modify how the final email look like here:
//builder.AddAttachment("c:\\report.txt");
//builder.Subject = "overwrite the subject";
IMail forwarded = builder.Create();
using (Smtp smtp = new Smtp())
{
smtp.Connect(MailConfig.smtpserver);
MailConfig.Connect(MailConfig.smtpuser, smtp);
ISendMessageResult result = smtp.SendMessage(forwarded);
}