Errors in mail.log from nagios check_ssmtp

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I got my nagios server banned by fail2ban because of errors in the postfix mail.log log. I know that I can simply whitelist the nagios server but I prefer it working perfectly.

Checking the logs I could see this error repeating itself on each check:

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Mar 25 13:01:13 xxx-123 postfix/smtpd[17065]: connect from nagios.example.com[1.2.3.4]
Mar 25 13:01:13 xxx-123 postfix/smtpd[17065]: improper command pipelining after QUIT from nagios.example.com[1.2.3.4]:
Mar 25 13:01:13 xxx-123 postfix/smtpd[17065]: disconnect from nagios.example.com[1.2.3.4]

Apparently postfix is picky about having extra input after a QUIT or DATA command, see details here.

It turns out that I haven’t updated nagios plugins in a while. Even if I kept nagios up-to-date the plugins were at 2.0.3. Updating to 2.1.1 fixed the issue and now I simply see a connect/disconnect in the postfix logs when nagios performs a check.