[Exim] Removing headers in ACLs

I was just wondering if anyone can tell me why the ability to remove headers
during the ACL processing stage doesn't exist (just wondering if it's
currently impossible due to certain constraints!).

It would seem to make some sense that if we're able to add headers at
certain stages of the ACL stuff, that we could also remove them at that
time. I'd assume as well that if we could do that, we would save a bit of
processing time since we remove it before the message is written to disk (I
think?), and wouldn't need to be modified later by a router or transport.
It would also help for those cases where people add stuff like
X-Authenticated type headers to store AUTH id's for email tracking, but want
to ensure that any X-Authenticated header is first removed from the incoming
message, so that the sender can't specify one themselves (which would be my
use for this). Oh and yeah I could probably do this with a system filter,
or at a router or transport layer, but I just think it would be nice to have
it possible in ACLs too.

I'm interested in this more in a source code programming sort of way, since
if it's just that nobody has got around to adding this feature, I might try
taking a stab at it :)

Eli.

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