How to check easily the munin alert-email-notification?

I run the munin monitoring application on my debian Wheezy server and I would like to test the alert-email-notification procedure. To do that, I add in munin.conf the following line: df._home.warning 25 I set 25 because the df command returns 30 for /home. I also defined one contact: contacts me contact.me.command mail "munin test ${var:host}" my_email@my_isp.com Unfortunately I don't receive any mails. There are no errors in the logs. Note that I use exim4 to send emails and that I can send emails from a terminal. What could be wrong in my configuration ? Note also I ckecked manually the df plugin: munin-run df returns 30 for /home