[exim] Dynamic "primary_hostname"

Hi all,

Wondering if there's a way to set "primary_hostname" to the sender's
domain (for local senders only, of course), whether in exim.conf or as
a command-line argument.

We have a dedicated box with 5 domains on it, and HELO/EHLO is working
as it should using the following config. The emails are notifications
from my PHP application (currently passing the "-f email@???"
argument to get envelope and return-path right).

--
interface = ${lookup{$sender_address_domain}lsearch{/etc/domainips_reverse}{$value}{$interface_address}}
helo_data = ${lookup{$interface_address}lsearch{/etc/domainips}{$value}{$primary_hostname}}
--

Is there a similar setting I could use for primary_hostname? I've
tried a lot of things, among them sender_host_name and
smtp_active_hostname, but it looks like most of these things are for
incoming connections. Please keep in mind that I'm not very familiar
with Exim expansion variables or conditions.

Would a statement setting primary_hostname to sender_address_domain if
sender_address_domain is part of a local list work?

This would solve all my problems. Otherwise I have to try to change
Received and message-id headers. Thanks very much for any insight,
and thanks for reading!

Todd