[Exim] Rewriting problem

I'm having a problem with address rewriting.

I'm trying to rewrite the address

    CURL@???

as
    helpdesk@???

Rewriting rule is

dbm;/usr/exim/tables/rewrite-addresses ${lookup{$local_part@$domain}dbm{/usr/exim/tables/rewrite-addresses}{$value}fail} Eh

and the relevant table entry is

curl@??? helpdesk@???


The problem is that I get different results depending on the case of the
username - ie

$ exim -brw curl@???
  sender: helpdesk@???
    from: helpdesk@???
      to: helpdesk@???
      cc: helpdesk@???
     bcc: helpdesk@???
reply-to: helpdesk@???
env-from: helpdesk@???
  env-to: helpdesk@???
$ exim -brw CURL@???
  sender: CURL@???
    from: CURL@???
      to: CURL@???
      cc: CURL@???
     bcc: CURL@???
reply-to: CURL@???
env-from: CURL@???
  env-to: CURL@???

I've tried putting the table entry as

CURL@??? helpdesk@???

but this makes no difference. We're running 3.36.

Has anyone got any ideas about how to sort this out?

Chris Bayliss
Information Services
The University of Birmingham