[Exim] Exim configuration error

Hi All,

complete and utter newbie here, so please go easy.

relaying mail from one server through another for internal net reasons. The
originating email address is user@hostname, changing the host name on the
originating machine is not an option either is being able to specify the
sender address. Trying to get exim to replace the email address to a valid
domain name. I have placed the following line within the REWRITE
CONFIGURATION section in the configure file.

headers_rewrite = user@hostname user@??? f :

when I try a bin/exim -C configure -bv

I get the following

domain missing in rewrite key string "headers_rewrite" in line 422

can anyone help at all,

many thanks

Leon.