I've been googling and reading mailing list posts until my eyes have gone buggy with little success:

Can anyone explain in step-by-step fashion how I configure EXIM to hide or remove the server's hostname from emails sent by my clients? I know this must be possible to do, but I haven't the foggiest.

I basically want email from my server to look like it is coming from the client's domain and not the server hostname.

Running Exim 4.x and RHE.

Thanks!
Neil

In general, you can't, that's how MTA's work, they specify the hostname of the server that they run on.

However, you can override this in Exim, by specifying trusted users, who can change various envelope parameters, including hostname and return-path using command line arguments. Have a read of this for further information:

http://www.exim.org/exim-html-4.30/doc/html/spec_5.html#SECT5.2

That makes sense. Where does this list of trusted users go, though? Into exim.conf? Or is there a separate file for this information?

Into exim.conf, at the top should do.

Can you give me an idea of what the syntax would look like? I've tried googling this for an hour or so but couldn't find any documentation or information on how to implement this...

No idea, I'm afraid. I've never used the option. To be honest, we've always educated our users on why it works the way it does.

In the main configuration section, add the users:

trusted_users = exim : user1 : user2 : user3