munin monitoring - email notification not delivered

I installed the munin monitoring tool on a Debian Wheezy server and its runs correctly excepted the email alert notification process. I configured munin.conf as requested and I added a test plugin generating warning alerts.

The /var/mail/munin file display this kind of message:

From MAILER-DAEMON Mon Feb 02 18:35:57 2015
Return-path: <>
Envelope-to: munin@debian
Delivery-date: Mon, 02 Feb 2015 18:35:57 +0100
Received: from Debian-exim by debian with local (Exim 4.80)
id 1YIKue-0007dm-Ky
for munin@debian; Mon, 02 Feb 2015 18:35:52 +0100
X-Failed-Recipients: my_email@my_isp.ch
Auto-Submitted: auto-replied
From: Mail Delivery System <Mailer-Daemon@debian>
To: munin@debian
Subject: Mail delivery failed: returning message to sender
Message-Id: <E1YIKue-0007dm-Ky@debian>
Date: Mon, 02 Feb 2015 18:35:52 +0100

This message was created automatically by mail delivery software.

A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:

my_email@my_isp.ch
SMTP error from remote mail server after MAIL FROM:<munin@debian> SIZE=1490:
host my_isp.ch [195.186.227.51]: 553 MAIL FROM:
<munin@debian> Unable to verify MX-Record for domain debian

------ This is a copy of the message, including all the headers. ------

Return-path: <munin@debian>
Received: from munin by debian with local (Exim 4.80)
(envelope-from <munin@debian>)
id 1YIKuK-0007dG-9D
for my_email@my_isp.ch; Mon, 02 Feb 2015 18:35:32 +0100
To: my_email@my_isp.ch
Subject: Munin notification localhost.localdomain
Message-Id: <E1YIKuK-0007dG-9D@debian>
From: munin@debian
Date: Mon, 02 Feb 2015 18:35:32 +0100

localdomain :: localhost.localdomain :: test plugin
WARNINGs: testValue is 2.10 (outside range [:2]).

I don't know how to escalate the problem. My isp seems to reject the email sent by munin.

Note that I can send emails from a terminal and that the logwatch application send correctly emails ! why not munin ?