To check whether Munin email notifications are still functioning, I have changed temporarily the "warning" and "critical" threshold to a very low value, and restarted the munin-node service.
To my dismay, I did not receive any Munin alert.
The Postfix service is running on port 25 and I can easily connect via telnet to it. I can send mail from command line -- and receive it correctly on my remote mailbox.
/var/log/maillog contains references to the messages I sent from command line but nothing from Munin, so it appears that Munin is not sending any alert.
Is there any log that I could peruse to see what's wrong?