[Exim] Problem with locally generated Mail

Hello,

I have a problem with locally generated mail.

I want that a user can send mail locally and provide a "From: " in the
header,but the user shouldn't be able to use any "From: " he wants,
only for domain he owns.

Background or motivation for that: i have multiple users running webpages
on the server and they want to send mail from PHP using custom "From: "
lines, since they all have domains and a own user (suexec enabled) it
would be fine if there is way to let them use whatever domain they own
in there "From: " part.


I had in mind something like this using an SQL query

SELECT 1 FROM domain_db WHERE domain_part='<the domain the user wants to
sent the message from>' AND user='<system user>'


If this is true it should be allowed, if not - maybe replace the
originalfrom
with something default (for that system user) or deny delivery (both is ok)

But I don't know where to include this in an ACL or any other condition,
any ideas would be great. I just can't find an appropriate rule for this
thing.

Thanks in advance,
Stefan