[Exim] Masquerade needed for outbound mail.

I've run into a problem that I didn't anticpate...

Apparently all of our unix hosts when they send mail out the mail server, have the senders email address masquerated as just the domain name.

So if the mail was from username@???, when it goes out it is username@???

The Domain.name is currently always the same.

send_to_smart_host:
driver = manualroute
domains = ! mailhost.domain.name
route_list = * mailhost.otherdomain.name
transport = remote_smtp
no_more

This is the end of the chain router.. so it takes everything the other routers didn't know about and smart hosts it to another server in another domain for delivery to the internet.

what do I need to put in the transport which currently looks like:

remote_smtp:
driver = smtp

Which I will change to (and update the above router to use)

smarthost_remote_smpt:
driver = smtp

so that the outbound from is changed if needed from

user@??? ->>> user@???

I assume it is a

headers_rewrite = ???

Never done rewriting before.

Or am I going to have to do something else.