[exim] mangling received headers in incoming mail.

I get email for three domain on four machines.
each one is listed as MX for the three domain, and each one has a list of
user of every domain.
Lower mx is the same host that has imap server and mailboxes for the
domain, the other do accept email in case of overload for the main one.
The fourth, that has no own domain, is used mainly to send e-mail for all.
each user has his .spamassassin/user_conf and is han dled by the machine
that has his mailbox.
With this arrangemente there is a problem: the secondary mx just refuse
e-mail for non-existing users, but pass all the other one to the main one.
The latter then cannot test on the receive headers for ill-behaving [aka:
spam sender] hosts.
Is there a way to telle exim not to add an header if the message is
received from one of secondary mx , that is now there is:
Received from mx10 by mx20 at ....
Received from mx20 from remote at ....

i want to have just
Received from mx20 from remote

Is there a way to get this (of course i can modify configuration file for
all the mx servers) ?
Alos, there is a way to make clamav test from the first machine that get
the message from exterior and skip on next step ?