Michael wrote:

> Thanks for your reply, Jakob. It sounds like I need to find a new
> provider because their MTA filtering is limited. Did I understand you
> to say that what I'm trying to do should normally not be difficult?

Yes, but nevertheless such a feature is not common in provider setups.
Maybe because most people don't request or know that's it's possible.

> That is, if I switch providers should I expect to be able to accomplish
> this? Better yet, what features should I be asking them for before I
> sign up?

"filtering on envelope data", "filterin at smtp time", "blacklist
recipients".

> I guess $message_headers doesn't work because of the order in which
> they are doing filtering?

$message_headers: This variable contains a concatenation of all the
header lines when a message is being processed, except for lines added
by routers or transports. The header lines are separated by newline
characters.

Since Envelope-To is added by the transport, it will not be in
$message_headers.
But as I said, it could be possible to filter on $header_received. OTOH,
it should then also appear in $message_headers.