[exim] How to rewrite a local from id

Several PHP scripts on my server send messages where the mail system
inserts an envelope header from apache@mydomain. I want to replace
"apache" with "postmaster" everywhere it may appear in a header. The
manual (exim-html-current) makes this sound trivial.

I have:

begin rewrite
*apache* {$1}postmaster{$2} Eh

That is it. I have nothing else I want to change.

When exim is run with -C (test config file) -brw apache@???, it
always returns "No rewrite rules are defined". If the above isn't a
rewrite rule, then I am totally lost.

I have tried several alternative rules, but no joy.

Any help appreciated.