This script sends a mail with a specific id in the subject via an given SMTP server to a given email-adress. When the script is run again, it checks for the delivery of this email based on its unique id in the subject on given POP3/IMAP mail accounts.
Using an external mail forward this script enables you to assert basic functionality of sending & receiving emails of your email infrastructure.
Example
check_email_loop -poph=host -pa=pw -popu=popts -smtph=host \
-from=root\@me.com -to=remailer\@testxy.com \
-lostc=0 -pendc=2
This is a plugin I wrote centuries ago. Meanwhile I do not run any email servers any more, but this plugin still seems to be very useful for a resonable share of the nagios users so that many other supplied significant improvements and extensions to this plugin. I'm very happy that my little script has become part of this lively open source community.
Though I wont' be able to further maintain this plugin on my own, I'm still trying my best to integrate code contributions I receive via email and share them at this place. So if you have a litte code change correction you feel it should find it's way into the public: just fork the GitHub repository and give me a short note.
Please also check the user comments on this plugin as they might contain more current information.