[exim] A simple rewriting problem?

I have been using exim happily for years but in the last week or two I
have hit a problem I do not understand.

The here LAN has about 7 computers on it that send mail -- all except
the android devices use exim. One machine (called snout) acts as a
mail server for the domain (codemist.co.uk). One of the other
computers (cage) when sending mail (from emacs) gives the From address
as jpff@cage with no domain, and one of the android devices that mails
to snout has From address as jpff@snout. At present I suppress all
local machine names with a rewrite

*@*.codemist.co.uk $1@??? bctFf

which seems sane. I tried a rewrite of
*@* but that captures too much.

So what have I done wrong I am sure I changed nothing) or even better how
do I fix it

Apologies for my stupidity
==John ffitch