hostname is being appended to the From name

[lists]

I'm trying to understand why this is happening and how to prevent it. I 
have a relay where if an email is sent to it with just a name in the 
Header From, then the server's hostname is added to the end of it. For 
example,  if I telnet to the server and send an email with "From:Test", 
then I'll get an email from Test@hostname.