[CentOS] Postfix (I think) problem

Sat Jan 24 15:10:03 UTC 2015
Timothy Murphy <gayleard at eircom.net>

I'm getting repeated email (KMail) error messages
about one apparently over-large post:
Received: from helen.gayleard.com (localhost.localdomain []) by 
helen.gayleard.com (Postfix) with ESMTP id E4500294A0 for 
<tim at helen.gayleard.com>; Sat, 24 Jan 2015 14:00:05 +0000 (GMT)
Received: (from tim at localhost) by helen.gayleard.com (8.14.4/8.14.4/Submit) 
id t0OE02Ie006372; Sat, 24 Jan 2015 14:00:02 GMT
Date: Sat, 24 Jan 2015 14:00:02 +0000
X-Authentication-Warning: helen.gayleard.com: tim set sender to root using -
From: Cron Daemon <root at helen.gayleard.com>
To: tim at helen.gayleard.com
Subject: Cron <tim at helen> /usr/bin/fetchmail -s mail.eircom.net

fetchmail: SMTP error: 552 5.3.4 Message size exceeds fixed limit
fetchmail: mail from MAILER-DAEMON at helen.gayleard.com bounced to 
biblioteca at comune.anghiari.ar.it
I've appended
  mailbox_size_limit = 20000000
to /etc/posfix/main.cf (and re-started postfix)
but this does not seem to have done the trick.

I get this email by fetchmail, and it goes to dovecot
from which I retrieve it on my laptop.
So I'm not entirely clear where the problem occurs.

I would delete the email if I could find it,
but it does not appear to be in my Maildir, or in /var/spool/ .

Is there some way I can stop these messages, please?

Timothy Murphy  
gayleard /at/ eircom.net
School of Mathematics, Trinity College, Dublin