what does this "sender invalid" error mean?

[Chris G]

I'm trying to b[ounce] a message and I'm getting the following error
on the mutt status line:-

     SMTP session failed: 550 Sender <chris@> invalid

So what is it saying?  Obviously "Chris@" id invalid but how is mutt
creating it and what is missing such that mutt can't create a valid
address? 

-- 
Chris Green