[exim] sender return-path mismatch again

Hello,

some time ago, I asked for a way for customers on a webserver so send
emails with an arbitrary sender-address. I have

untrusted_set_sender = *
local_from_check = false

in my configuration right now. If someone calls

sendmail -t -flehmann@???
From: lehmann@???
To: lehmann@???
Subject: test

this mail indead has the sender-address and return-path lehmann@???.
But this -f option is too complicated to use because it is unknown to
the most users. Most of them are just using the mail()-function in PHP
and don't care about return-path and sender-address. And I guess you
can't blame them for this. What I'm still looking for is a way to tell
exim to use the address stated in the "From:" line as the return-path.
If there's no such line, exim can still use uid@hostname. As the sending
uid is stored in the message header anyway, there should be a problem
with unknown users or disguised identities. How do other webhosting
provider solve this? The way I described used to work with qmail, but I
don't want to install qmail again.

Regards
Marten