Can you post the code from the receiver?
See if this change helps:
using System.IO.Pipes;
...
string mail = "/MAIL USER=MailBox;TO=box@yahoo.com;SEND";
System.Text.Encoding enc = System.Text.Encoding.ASCII;
byte[] buff = enc.GetBytes(mail);
NamedPipeClientStream pipe = new NamedPipeClientStream("The Bat!"); pipe.Connect();
pipe.Write(buff, 0, buff.Length);
pipe.Flush();
pipe.Close();
...