How to set envelope sender (MAIL FROM) using exim

[Longman]

* Thomas Pircher wrote:
> Ups, I think I have misunderstood you. Do you mean the user on the other
> side presses the "reply" button and his mail program takes the return-path
> instead of the From: address?

Yes.  Well it doesn't actually reply per se.  I'll explain. The email
address that I send to generates a text message (it's T-Mobile's email
to SMS gateway) and in the generated SMS they put the email address and
the contents of the email body in ASCII.  I use it to give my N95
bespoke push-email-esque behaviour ;-).  When I receive an email it goes
through a procmail rule that generates a timestamp and sends this as the
body text of an email to the gateway which then ends up on my phone as
an SMS. I then know I have to check my email server at some point since
there is email there.  For some reason in the SMS that T-Mobile generate
 gateway service uses the return-path from the email they receive rather
than the From address to say who the email is from.  So it's a reply,
but not an email reply and their system is picking up the return-path
despite me setting a valid From.

Your suggestion of modifying /etc/mail-addresses has allowed me to get
the correct return-path set on outbound emails and it now works. A
Reply-To header may work (I might just try it to see anyway) but since
the email is converted and forwarded to SMS rather than replied to with
email semantics I'm not sure their system would take note of this header?