I usually receive this mails from the smtp server.
I think that logcheck should check the size of emails and split
in some of them if contents exceed 5MB or something like that.

A message that you sent was longer than the maximum size allowed on this
system. It was not delivered to any recipients.
------ This is a copy of your message, including all the headers.
------ No more than 100K characters of the body are included.

To: logcheck
To: logcheck@goliat.ls.fi.upm.es
Subject: ... 2009-05-12 00:00 System Events
Message-Id: <E1M3f80-0003OZ-Ea@...>
From: logcheck system account <logcheck@...>
Date: Tue, 12 May 2009 01:41:48 +0200

This email is sent by logcheck. If you wish to no-longer receive it,
you can either deinstall the logcheck package or modify its
configuration file (/etc/logcheck/logcheck.conf).

...