[exim] Sender/envelope-from rewrite question

I am seeing that mail sent by an authenticated user over a submission
port is showing the sender and envelope-from as
"username@???

I read the documentation on rewriting and would like to do it based on
a list of authenticated users so I have added this in acl_check_rcpt:

accept  authenticated = *
          control       = submission/domain=example.com/\
                                  name=${lookup {$authenticated_id} \
                                          lsearch {/etc/mail/namelist}}

This has partly fixed the problem. Now the sender is
"user@???

How can I get this to be username@????

-Kevin