Strange messages with 1.61

I have not been on the list for quite awhile.

I have started using exim as a mail exploder for the mailing lists and I
have two systems running right now

1. Linux 2.0.30 on a Cyrix 166 forwarding around 60.000 messages

2. Linux 2.0.29 on a 486DX66 with around 50.000 / day

I frequently get messages frozen on both systems. HEre is one of the
examples I get from exim when that happens:

What is this errormessage? What does it mean and how can I stop this
from happening.

Date: Sun, 27 Apr 1997 06:44:00 -0700
From: Mail Delivery System <Mailer-Daemon@???>
To: postmaster@???
Subject: Message frozen

Message 0wLUCD-0002C6-00 has been frozen. The sender is
<owner-linux-newbie-outgoing@???>.

The following address(es) have yet to be delivered:
Bretter@???: smtp transport process returned a non-zero
status (0x000b)
JPHAHN@???: smtp transport process returned a non-zero
status (0x000b)
drkjster@???: smtp transport process returned a non-zero
status (0x000b)
no0ne@???: smtp transport process returned a non-zero status
(0x000b)