On a recently installed machine, the Nagios check for mailq did not work. Instead it returned the following error:

This is on a QMail based machine running on FreeBSD. I compared the setup with an older FreeBSD machine, where the check works properly. This box was also running QMail but with an older set of Nagios plugins. The configuration was equal, except the check_mailq script itself, because it has evolved a little in the meantime.

According to one bug wrangler on Gentoo Bugzilla, you're a stupid person if you don't read the messages spitted out by ebuilds and run into failures because of that. Of course, no sane person watches the complete compilations of application on the lookout for warnings or other informative messages. But it would be nice if you get a nice summary of all messages in your mailbox after an emerge command.

QMail is quite a challenge to set up right, this will take you hours if you don't install it on a weekly basis. When you closed all holes from the outside to prevent acting like a open relay, there is the security on the localhost itself. It happened a few times already that a PHP script had an impromper loop condition which had a mail() call inside. This will guarantee you for having 10.000s of mails in your queue. Or think of a leak in a contact form abused by spammers.