A cron daemon is a utility used to automate things on Unix-based operating systems. One such task they may run is sending emails that people subscribed to. Unfortunately, every so often a programmer gets their code (PHP code for example) a little mixed up and this will cause two or more emails to be sent to you. This oftentimes provides a spectacular way to overload your Gmail inbox or at the minimum, get very annoying, very quickly because now you have all the extra emails to get in your way.
Since these kinds of utilities don't always have a user interface that you can use to unsubscribe yourself, it can be impossible to get off of email lists sent from cron jobs if you don't have access to the server that sends the emails.
If you need to block emails like this, follow these instructions:
Install Block Sender for Gmail by clicking the “Install” button found in the upper right-hand corner of this site. The installation won’t take long. Open Gmail when it is done.
After signing up for a Block Sender account and linking your Gmail account to Block Sender, open the unwanted email from the cron daemon, and click the "Block" button on the messages toolbar.
Select the "Emails from this sender" option.