[Exim] Reject mail based on MAIL FROM from envelope

Hi

I want to reject all posts when MAIL FROM contains
space for example:
MAIL FROM: <user1@??? >
MAIL FROM: <user2@??? >
MAIL FROM: <user3@??? >
                             ^-this space

I want to do this at the begining of the SMTP session

$ telnet smtp 25
220 smtp.smt.pl ESMTP Exim 3.36 #1 Mon, 29 Apr 2002 16:14:32 +0200
mail from: <aaa@??? >
250 <aaa@???> is syntactically correct

I want to obtain:
550 rejected: administrative prohibition
instead of 250 <aaa@???> is syntactically correct

Arek