[Exim] exim 3.16, procmail and mail delivery failures

Hello,

I`ve created a test user with .forward file containing: |/usr/bin/procmail
and .procmailrc :
:0 c
|/path/to/home/test/forwarding_script

here`s the script:

#!/bin/sh
EMAIL=anothertest@myhost
(cat - | formail -X From: -X Subject:
echo To: \<$EMAIL\>
echo
echo .
echo quit
) | /usr/sbin/exim -t

Now the problem :)
Sometimes I get delivery failure reports containing error:

procmail: Error while writing to "/path/to/home/test/forwarding_script"

I`ve noticed that this error occurs sometimes - mainly when I try to send e-mail with an attachment (but it`s not the rule).
What can be wrong ? Any ideas ?

best regards,
Daniel Rycaj